Impelment DataFrame.keys

[itholic]

An alias for columns.

>>> df = ks.DataFrame([[1, 2], [4, 5], [7, 8]],
...                   index=['cobra', 'viper', 'sidewinder'],
...                   columns=['max_speed', 'shield'])
>>> df
            max_speed  shield
cobra               1       2
viper               4       5
sidewinder          7       8

>>> df.keys()
Index(['max_speed', 'shield'], dtype='object')

>>> df.to_pandas().keys()
Index(['max_speed', 'shield'], dtype='object')

[ueshin]

Otherwise, LGTM.

[HyukjinKwon]

LGTM too except https://github.com/databricks/koalas/pull/937/files#r336500413

[itholic]

@ueshin @HyukjinKwon Thanks for commenting!! added it :)

[softagram-bot]

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[codecov-io]

Codecov Report

Merging #937 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #937      +/-   ##
==========================================
+ Coverage   94.46%   94.46%   +<.01%     
==========================================
  Files          34       34              
  Lines        6382     6383       +1     
==========================================
+ Hits         6029     6030       +1     
  Misses        353      353

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
databricks/koalas/frame.py	96.02% <100%> (ø)	⬆️

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