Fix spark_df when set_index(.., drop=False).

[ueshin]

No description provided.

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Softagram Impact Report for pull/792 (head commit: a9cbbd3)

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[codecov-io]

Codecov Report

Merging #792 into master will decrease coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #792      +/-   ##
==========================================
- Coverage   94.28%   94.27%   -0.01%     
==========================================
  Files          32       32              
  Lines        5770     5768       -2     
==========================================
- Hits         5440     5438       -2     
  Misses        330      330

Impacted Files	Coverage Δ
databricks/koalas/internal.py	95.85% <80%> (-0.04%)	⬇️

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[HyukjinKwon]

ah, thanks. LGTM

[ueshin]

Thanks! merging.