A colorbar label doesn't automatically get created for labelled dataset for bokeh backend

[ahuang11]

I vaguely remember that the clabel automatically gets created a while back, so I think this might be a regression.

In matplotlib, it automatically shows "z"

import numpy as np
import holoviews as hv
hv.extension('matplotlib')

ls  = np.linspace(0, 10, 400)
x,y = np.meshgrid(ls, ls)
img = hv.Image(np.sin(x)*np.cos(y)+0.1*np.random.rand(400,400), 
               bounds=(-20,-20,20,20)).opts(colorbar=True, xaxis=None, yaxis=None)

hv.Layout([img.relabel(c).opts(cmap=c) for c in ['gray','PiYG','flag','Set1']])

Changing it to bokeh, the clabel no longer exists.

Another example:

import hvplot.xarray
import xarray as xr
import holoviews as hv
hv.extension("bokeh")

ds = xr.tutorial.open_dataset("air_temperature").isel(time=0)
hv.Image(ds, ["lon", "lat"], ["air"]).opts(colorbar=True).redim.label(lat="Latitude", lon="Longitude", air="Air Temperature")