fps keyword doesn't work with scrubber

#### ALL software version info
(this library, plus any other relevant software, e.g. bokeh, python, notebook, OS, browser, etc)

hvplot version 0.5.2.

#### Description of expected behavior and the observed behavior

The `fps` keyword does not work when creating an xarray.hvplot with a scrubber.

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

```
import xarray as xr
import hvplot.xarray # noqa

air_ds = xr.tutorial.open_dataset('air_temperature').load()
air = air_ds.air
air.hvplot(groupby='time', width=600, widget_type='scrubber', widget_location='bottom',
fps=5)
```

Gives the warning

```
WARNING:param.main: fps option not found for image plot; similar options include: []
```