Fixed DataFrame widget datetime formatting

[philippjfr]

Fixes #1195

[codecov]

Codecov Report

Merging #1221 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1221      +/-   ##
==========================================
+ Coverage   87.41%   87.43%   +0.02%     
==========================================
  Files         106      106              
  Lines       12619    12627       +8     
==========================================
+ Hits        11031    11041      +10     
+ Misses       1588     1586       -2     

Impacted Files	Coverage Δ
panel/tests/widgets/test_tables.py	95.94% <100.00%> (+0.49%)	⬆️
panel/widgets/tables.py	79.50% <100.00%> (+1.63%)	⬆️

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