Fix param mapping for param.CalendarDate and DatePicker

[philippjfr]

Fixes #1271

[codecov]

Codecov Report

Merging #1434 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1434      +/-   ##
==========================================
+ Coverage   87.90%   87.93%   +0.03%     
==========================================
  Files         135      135              
  Lines       14854    14879      +25     
==========================================
+ Hits        13057    13084      +27     
+ Misses       1797     1795       -2     

Impacted Files	Coverage Δ
panel/param.py	91.55% <ø> (ø)
panel/widgets/input.py	96.12% <100.00%> (ø)
panel/tests/widgets/test_button.py	100.00% <0.00%> (ø)
panel/links.py	85.56% <0.00%> (+1.05%)	⬆️
panel/widgets/button.py	85.71% <0.00%> (+2.38%)	⬆️

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