Fix fillna to handle NaN properly.

[ueshin]

Series.fillna should handle NaN properly.

>>> kser = ks.Series([1, 2, 3, 4, 5, 6], name="x")
>>> kser.loc[0] = np.nan
>>> kser
0    NaN
1    2.0
2    3.0
3    4.0
4    5.0
5    6.0
Name: x, dtype: float64
>>> kser.fillna(0)
0    NaN
1    2.0
2    3.0
3    4.0
4    5.0
5    6.0
Name: x, dtype: float64

This should be:

>>> kser.fillna(0)
0    0.0
1    2.0
2    3.0
3    4.0
4    5.0
5    6.0
Name: x, dtype: float64

[codecov-commenter]

Codecov Report

Merging #1572 into master will increase coverage by 0.00%.
The diff coverage is 88.88%.

@@           Coverage Diff           @@
##           master    #1572   +/-   ##
=======================================
  Coverage   94.54%   94.54%           
=======================================
  Files          38       38           
  Lines        8760     8763    +3     
=======================================
+ Hits         8282     8285    +3     
  Misses        478      478           

Impacted Files	Coverage Δ
databricks/koalas/series.py	97.58% <88.88%> (+<0.01%)	⬆️

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