Fix concat for multi-index columns support.

[ueshin]

No description provided.

[codecov-io]

Codecov Report

Merging #680 into master will decrease coverage by 0.01%.
The diff coverage is 94.87%.

@@            Coverage Diff             @@
##           master     #680      +/-   ##
==========================================
- Coverage   93.54%   93.52%   -0.02%     
==========================================
  Files          32       32              
  Lines        5456     5455       -1     
==========================================
- Hits         5104     5102       -2     
- Misses        352      353       +1

Impacted Files	Coverage Δ
databricks/koalas/internal.py	96.15% <ø> (-0.04%)	⬇️
databricks/koalas/namespace.py	89.38% <100%> (+0.18%)	⬆️
databricks/koalas/indexing.py	92.43% <71.42%> (-0.64%)	⬇️

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[ueshin]

Thanks! merging.