Fix GroupBy.fillna to use GroupBy._apply_series_op.

[ueshin]

GroupBy.fillna can also use GroupBy._apply_series_op instead of its own logic to collect Series.

[codecov-io]

Codecov Report

Merging #1481 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1481      +/-   ##
==========================================
- Coverage   93.80%   93.79%   -0.01%     
==========================================
  Files          36       36              
  Lines        8440     8430      -10     
==========================================
- Hits         7917     7907      -10     
  Misses        523      523              

Impacted Files	Coverage Δ
databricks/koalas/groupby.py	89.17% <100.00%> (-0.19%)	⬇️

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