Fix pivot_table for multi-index columns support.

[ueshin]

No description provided.

[codecov-io]

Codecov Report

Merging #908 into master will increase coverage by 1.56%.
The diff coverage is 98.18%.

@@            Coverage Diff            @@
##           master    #908      +/-   ##
=========================================
+ Coverage   92.73%   94.3%   +1.56%     
=========================================
  Files          34      34              
  Lines        6265    6286      +21     
=========================================
+ Hits         5810    5928     +118     
+ Misses        455     358      -97

Impacted Files	Coverage Δ
databricks/koalas/testing/utils.py	78.39% <100%> (+0.4%)	⬆️
databricks/koalas/frame.py	95.98% <98.03%> (+0.04%)	⬆️
databricks/conftest.py	97.87% <0%> (+4.25%)	⬆️
databricks/koalas/__init__.py	85% <0%> (+7.5%)	⬆️
databricks/koalas/usage_logging/usage_logger.py	100% <0%> (+50%)	⬆️
databricks/koalas/usage_logging/__init__.py	97.27% <0%> (+72.72%)	⬆️

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