Support MultiIndex for DataFrame.duplicated, drop_duplicates.

[ueshin]

Currently DataFrame.duplicated and DataFrame.drop_duplicates don't support MultiIndex.
We should support them.

e.g.,

>>> pdf
          a  b  c
0.660073  1  1  1
0.255808  1  1  1
0.796535  2  1  1
0.562986  3  4  5
>>> pdf.duplicated()
0.660073    False
0.255808     True
0.796535    False
0.562986    False
dtype: bool
>>> pdf.set_index("a", append=True).duplicated()
          a
0.660073  1    False
0.255808  1     True
0.796535  2     True
0.562986  3    False
dtype: bool

or

>>> pdf.drop_duplicates()
          a  b  c
0.660073  1  1  1
0.796535  2  1  1
0.562986  3  4  5
>>> pdf.set_index("a", append=True).drop_duplicates()
            b  c
         a
0.660073 1  1  1
0.562986 3  4  5

[codecov-io]

Codecov Report

Merging #1363 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1363      +/-   ##
=========================================
- Coverage   95.22%   95.2%   -0.02%     
=========================================
  Files          34      34              
  Lines        7705    7699       -6     
=========================================
- Hits         7337    7330       -7     
- Misses        368     369       +1

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.73% <100%> (-0.02%)	⬇️
databricks/koalas/generic.py	97.13% <0%> (-0.41%)	⬇️

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[itholic]

LGTM.

(FYI: Although we have some problem in Travis now, seems okay to just merge this since GitHub Actions is passed)

[ueshin]

Thanks! Let me merge this for now since Github Action builds passed.