Add index_col argument to to_koalas().

[ueshin]

This is related to #765.

[softagram-bot]

Softagram Impact Report for pull/863 (head commit: 273a793)

⭐ Change Overview

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💡 Insights

• Co-change Alert: You modified frame.py. Often test_dataframe.py (koalas/tests) is modified at the same time.

📄 Full report

• Permalink: Full report for pull/863

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[codecov-io]

Codecov Report

Merging #863 into master will decrease coverage by 1.29%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #863     +/-   ##
=========================================
- Coverage   94.31%   93.02%   -1.3%     
=========================================
  Files          34       34             
  Lines        6188     6705    +517     
=========================================
+ Hits         5836     6237    +401     
- Misses        352      468    +116

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.16% <100%> (+0.05%)	⬆️
databricks/koalas/usage_logging/__init__.py	24.54% <0%> (-72.73%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50% <0%> (-50%)	⬇️
databricks/koalas/__init__.py	77.5% <0%> (-7.5%)	⬇️
databricks/conftest.py	93.61% <0%> (-4.26%)	⬇️

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