Fixed DataFrame widget selection parameter

[philippjfr]

Fixes #923

[codecov]

Codecov Report

Merging #989 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #989      +/-   ##
==========================================
+ Coverage   85.79%   85.83%   +0.03%     
==========================================
  Files          99       99              
  Lines       11246    11260      +14     
==========================================
+ Hits         9649     9665      +16     
+ Misses       1597     1595       -2

Impacted Files	Coverage Δ
panel/tests/widgets/test_tables.py	95.38% <100%> (+1.15%)	⬆️
panel/widgets/tables.py	77.68% <100%> (+1.85%)	⬆️

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