fps keyword doesn't work with scrubber

[rabernat]

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: []