Ensure DatePicker start/end are transformed when jslinked

[philippjfr]

Fixes #1810

[codecov]

Codecov Report

Merging #3759 (42364f2) into master (df6eb4f) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3759      +/-   ##
==========================================
- Coverage   84.22%   84.20%   -0.03%     
==========================================
  Files         213      213              
  Lines       31018    31021       +3     
==========================================
- Hits        26124    26120       -4     
- Misses       4894     4901       +7     

Flag	Coverage Δ
ui-tests	33.98% <0.00%> (-0.02%)	⬇️
unitexamples-tests	77.11% <100.00%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
panel/widgets/input.py	96.05% <100.00%> (+0.02%)	⬆️
panel/io/reload.py	69.56% <0.00%> (-2.18%)	⬇️
panel/tests/util.py	85.14% <0.00%> (-1.99%)	⬇️
panel/io/cache.py	81.98% <0.00%> (-0.46%)	⬇️
panel/util.py	86.41% <0.00%> (-0.38%)	⬇️
panel/io/server.py	76.35% <0.00%> (-0.21%)	⬇️

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