Aggregating lines with column aggregator breaks on cuDF

When trying to perform line aggregation on a cuDF and specify an aggregator with a column you get an error:

```python
import cudf
import datashader as ds

nyc_df = cudf.read_parquet('~/Downloads/nyc_taxi_wide.parq')

cvs = ds.Canvas()

cvs.line(nyc_df, ['pickup_x', 'pickup_y'], ['dropoff_x', 'dropoff_y'], axis=1, agg=ds.mean('trip_distance'))
```

```
~/development/datashader/datashader/compiler.py in (.0)
96
97 def make_info(cols):
---> 98 return lambda df: tuple(c.apply(df) for c in cols)
99
100

~/development/datashader/datashader/reductions.py in apply(self, df)
36 else:
37 nullval = 0
---> 38 return cupy.array(df[self.column].to_gpu_array(fillna=nullval))
39 else:
40 return df[self.column].values

~/miniconda3/envs/rapidsai/lib/python3.7/site-packages/cupy/creation/from_data.py in array(obj, dtype, copy, order, subok, ndmin)
39
40 """
---> 41 return core.array(obj, dtype, copy, order, subok, ndmin)
42
43

cupy/core/core.pyx in cupy.core.core.array()

cupy/core/core.pyx in cupy.core.core.array()

cupy/core/core.pyx in cupy.core.core._convert_object_with_cuda_array_interface()

TypeError: zip argument #2 must support iteration
```

cupy version: '6.3.0'
cudf version: '0.10.0'