Fixed deserialization of polygon/multi_line CDS data in bokeh

[philippjfr]

Fixes #4626

[kcpevey]

Confirmed I no longer get the error with this fix. Thank you!!

The error was:

VM10348:52 Python failed with the following traceback: 
/Users/kcp/.conda/envs/lbm/lib/python3.8/site-packages/pyviz_comms/__init__.py _handle_msg L316
/Users/kcp/.conda/envs/lbm/lib/python3.8/site-packages/holoviews/plotting/bokeh/callbacks.py on_msg L146
/Users/kcp/.conda/envs/lbm/lib/python3.8/site-packages/holoviews/plotting/bokeh/callbacks.py _process_msg L1203
/Users/kcp/.conda/envs/lbm/lib/python3.8/site-packages/holoviews/plotting/bokeh/callbacks.py <listcomp> L1203
	ValueError: invalid literal for int() with base 10: 'dtype'