Fix passing options as keyword arguments

[ueshin]

Resolves #967.

[rxin]

This is embarrassing :( We should add a test case.

[ueshin]

Yup, let me add some.

[codecov-io]

Codecov Report

Merging #968 into master will increase coverage by 0.01%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #968      +/-   ##
==========================================
+ Coverage   94.55%   94.57%   +0.01%     
==========================================
  Files          34       34              
  Lines        6483     6483              
==========================================
+ Hits         6130     6131       +1     
+ Misses        353      352       -1

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.03% <ø> (ø)	⬆️
databricks/koalas/namespace.py	87.14% <60%> (+0.31%)	⬆️

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[softagram-bot]

Softagram Impact Report for pull/968 (head commit: 7be55ad)

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

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