Fix handling of 3-channel RGB element

[philippjfr]

The hv.RGB constructor adds the alpha channel if needed so it should not be declared in the parameter definition.

Fixes #522

[stas-sl]

Looks like after this change the following code stopped working:

import numpy as np
import geoviews as gv
from holoviews.operation.datashader import datashade

gv.extension('bokeh')

datashade(gv.Points(np.random.randn(100000, 2)))

It throws

The coordinates on the 'R' DataArray do not match the provided key dimensions (kdims). The following coords were left unspecified: ['band']. If you are requesting a lower dimensional view such as a histogram cast the xarray to a columnar format using the .to_dataframe or .to_dask_dataframe methods before providing it to HoloViews.

But if I revert this change, everything works well.

[philippjfr]

Damn, thanks for reporting that. We'll have to follow up with another release.