Fix drop for multi-index columns support.

[ueshin]

No description provided.

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Softagram Impact Report for pull/658 (head commit: 3f9bec5)

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[codecov-io]

Codecov Report

Merging #658 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #658      +/-   ##
=========================================
+ Coverage   93.19%   93.2%   +<.01%     
=========================================
  Files          32      32              
  Lines        5294    5300       +6     
=========================================
+ Hits         4934    4940       +6     
  Misses        360     360

Impacted Files	Coverage Δ
databricks/koalas/frame.py	94.48% <100%> (+0.02%)	⬆️

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[ueshin]

Thanks! merging.