rasterize and colormap doesn't return a plot

#### Description of expected behavior and the observed behavior
The following code does not return any plot. The code works if changing `rasterize` to `datashade`. I think this should atleast return an error message which advises the user to use `datashade` instead of `rasterize`.

#### Complete, minimal, self-contained example code that reproduces the issue

``` python
import datashader as ds
import hvplot.pandas
from bokeh.sampledata.iris import flowers as df

color_key = {"setosa": "red", "versicolor": "blue", "virginica": "green"}

df.hvplot.scatter(
x="sepal_length",
y="sepal_width",
rasterize=True,
colormap=color_key,
)
```

#### Stack traceback and/or browser JavaScript console output
``` console
Uncaught (in promise) TypeError: d.slice is not a function
cat_v_compute http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:22058
_v_compute http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:22018
v_compute http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:19879
_flat_img_to_buf8 http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:32151
_set_data http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:32236
set_data http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:20694
set_data http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:20057
lazy_initialize http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:19991
_build_view http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:12765
build_views http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:12783
build_renderer_views http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:37116
lazy_initialize http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:36895
_build_view http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:12765
build_views http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:12783
build_child_views http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:35439
lazy_initialize http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:35386
_build_view http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:12765
build_view http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:12769
render_model http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:43962
add_document_standalone http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:43978
embed_items_notebook http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:44651
embed_document http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:858
http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:870
http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:886
attachWidget http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
insertWidget http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_insertOutput http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
onModelChanged http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
m http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
l http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
emit http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_onListChanged http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
m http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
l http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
emit http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
push http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_add http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
add http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_onIOPub http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_handleIOPub http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
ian_help.ipynb line 2 > injectedScript:22058:37
add_document_standalone http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:43990
AsyncFunctionThrow self-hosted:672
(Async: async)
embed_items_notebook http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:44651
embed_document http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:858
http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:870
http://localhost:8888/lab/tree/development/hvplot/archive/ian_help.ipynb line 2 > injectedScript:886
attachWidget http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
insertWidget http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_insertOutput http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
onModelChanged http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
m http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
l http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
emit http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_onListChanged http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
m http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
l http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
emit http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
push http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_add http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
add http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_onIOPub http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_handleIOPub http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
AsyncFunctionNext self-hosted:668
(Async: async)
handleMsg http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_handleMessage http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_msgChain http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
(Async: promise callback)
_onWSMessage http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
(Async: EventHandlerNonNull)
_createSocket http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
(Async: setTimeout handler)
_reconnect http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
reconnect http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
restart http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
InterpretGeneratorResume self-hosted:1458
AsyncFunctionNext self-hosted:668
(Async: async)
restartKernel http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
restart http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
InterpretGeneratorResume self-hosted:1458
AsyncFunctionNext self-hosted:668
(Async: async)
restartKernelAndClear http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
r http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
execute http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
triggerActiveItem http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_evtMouseUp http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
handleEvent http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
(Async: EventListener.handleEvent)
onBeforeAttach http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
processMessage http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
b http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
t http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
r http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
c http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
open http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_openChildMenu http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
_evtMouseMove http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
handleEvent http://localhost:8888/static/lab/jlab_core.9193dfb13484acaca919.js?v=9193dfb13484acaca919:2
```

#### Screenshots or screencasts of the bug in action