Fix where to support multi-index columns.

[ueshin]

No description provided.

[codecov-io]

Codecov Report

Merging #1249 into master will increase coverage by 1.34%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master    #1249      +/-   ##
==========================================
+ Coverage   93.73%   95.08%   +1.34%     
==========================================
  Files          35       35              
  Lines        7154     7155       +1     
==========================================
+ Hits         6706     6803      +97     
+ Misses        448      352      -96

Impacted Files	Coverage Δ
databricks/koalas/base.py	97.29% <100%> (+0.9%)	⬆️
databricks/koalas/strings.py	98.37% <100%> (ø)	⬆️
databricks/koalas/frame.py	96.71% <100%> (-0.12%)	⬇️
databricks/koalas/namespace.py	87.87% <66.66%> (+0.3%)	⬆️
databricks/koalas/plot.py	93.33% <0%> (-0.96%)	⬇️
databricks/koalas/internal.py	95.94% <0%> (+0.36%)	⬆️
databricks/conftest.py	96.22% <0%> (+3.77%)	⬆️
databricks/koalas/__init__.py	85.1% <0%> (+6.38%)	⬆️
... and 2 more

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