Fix cumsum and cumprod.

[ueshin]

Fixes DataFrame/Series/GroupBy.cumsum and cumprod.

[codecov-io]

Codecov Report

Merging #1982 (b17d0da) into master (dd9661f) will increase coverage by 0.00%.
The diff coverage is 91.30%.

@@           Coverage Diff           @@
##           master    #1982   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files          50       50           
  Lines       10946    10952    +6     
=======================================
+ Hits        10352    10358    +6     
  Misses        594      594           

Impacted Files	Coverage Δ
databricks/koalas/groupby.py	91.58% <ø> (ø)
databricks/koalas/series.py	96.93% <89.47%> (+0.01%)	⬆️
databricks/koalas/generic.py	92.15% <100.00%> (ø)

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[HyukjinKwon]

LGTM