Fix drop_duplicates for multi-index columns support.

[ueshin]

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Softagram Impact Report for pull/856 (head commit: 51be507)

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[codecov-io]

Codecov Report

Merging #856 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #856      +/-   ##
==========================================
+ Coverage   94.22%   94.22%   +<.01%     
==========================================
  Files          34       34              
  Lines        6147     6150       +3     
==========================================
+ Hits         5792     5795       +3     
  Misses        355      355

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.1% <100%> (ø)	⬆️

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