Handle integer column names on Perspective widget

[philippjfr]

Fixes #2054

[hoxbro]

Do this change work with a dictionary like {0: [1,2,3], "0": [4,5,6]}?

If not, a check to _get_data to see that there is no overlapping dictionary keys like "0" and 0 is properly a good thing.

[philippjfr]

Yes, we should check there's no overlap.

[codecov]

Codecov Report

Merging #2069 (3f18137) into master (8a233ca) will increase coverage by 0.01%.
The diff coverage is 91.93%.

@@            Coverage Diff             @@
##           master    #2069      +/-   ##
==========================================
+ Coverage   84.43%   84.44%   +0.01%     
==========================================
  Files         178      179       +1     
  Lines       20636    20704      +68     
==========================================
+ Hits        17423    17484      +61     
- Misses       3213     3220       +7     

Impacted Files	Coverage Δ
panel/pane/perspective.py	91.80% <88.09%> (+2.51%)	⬆️
panel/tests/pane/test_perspective.py	100.00% <100.00%> (ø)
panel/auth.py	39.51% <0.00%> (-0.37%)	⬇️
panel/viewable.py	72.38% <0.00%> (ø)
panel/tests/test_reactive.py	98.90% <0.00%> (+0.01%)	⬆️
panel/widgets/base.py	94.66% <0.00%> (+0.07%)	⬆️

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