Reduce usages of data_columns.

[ueshin]

With this change, the following DataFrame APIs support multi-index columns:

• applymap
• shift
• diff
• fillna
• rank

[codecov-io]

Codecov Report

Merging #793 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #793      +/-   ##
==========================================
+ Coverage   94.28%   94.28%   +<.01%     
==========================================
  Files          32       32              
  Lines        5770     5773       +3     
==========================================
+ Hits         5440     5443       +3     
  Misses        330      330

Impacted Files	Coverage Δ
databricks/koalas/series.py	95.16% <100%> (+0.02%)	⬆️
databricks/koalas/frame.py	96.89% <100%> (ø)	⬆️
databricks/koalas/internal.py	95.85% <0%> (-0.04%)	⬇️

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[softagram-bot]

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[ueshin]

Thanks! merging.