Hot-fixing the lack of keyword argument 'deep' for DataFrame.copy()

[itholic]

Resolves #1422

Adding a dummy keyword argument deep which is doing nothing for DataFrame.copy(), since we doesn't support keyword deep for the DataFrame.copy().

[ueshin]

SGTM.
Shall we add it for Series and Index as well?
Also, could you add parameter sections in each function docstring to describe the argument is a dummy?

[itholic]

@ueshin

Thanks for the comment. I think it is better to do that, either.

[ueshin]

LGTM, pending tests.

[codecov-io]

Codecov Report

Merging #1423 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1423   +/-   ##
=======================================
  Coverage   95.13%   95.13%           
=======================================
  Files          34       34           
  Lines        7958     7958           
=======================================
  Hits         7571     7571           
  Misses        387      387           

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.61% <100.00%> (ø)
databricks/koalas/indexes.py	97.06% <100.00%> (ø)
databricks/koalas/series.py	96.92% <100.00%> (ø)

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