Compatibility with Bokeh 2 DatePicker

[hyamanieu]

See bokeh/bokeh#9509 , date is in ISO format.

[philippjfr]

Thanks, we'll also have to update the existing tests which mocks a change in the date but still uses the old date format

panel/panel/tests/widgets/test_input.py

Line 42 in ffcdd3c

date_picker._comm_change({'value': 'Mon Sep 03 2018'})

[codecov]

Codecov Report

Merging #1187 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1187   +/-   ##
=======================================
  Coverage   87.79%   87.79%           
=======================================
  Files         105      105           
  Lines       12598    12598           
=======================================
  Hits        11061    11061           
  Misses       1537     1537           

Impacted Files	Coverage Δ
panel/tests/widgets/test_input.py	100.00% <100.00%> (ø)
panel/widgets/input.py	96.12% <100.00%> (ø)

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[philippjfr]

Thanks again! Hugely appreciated.