Fix fillna not to change index values.

[ueshin]

DataFrame.fillna() should not change the index values.

>>> import pandas as pd
>>> import numpy as np
>>> pdf = pd.DataFrame({'x': [np.nan, 2, 3, 4, np.nan, 6],
...                     'y': [1, 2, np.nan, 4, np.nan, np.nan],
...                     'z': [1, 2, 3, 4, np.nan, np.nan]}).set_index(['x', 'y'])
>>>
>>> pdf.fillna(-1)
           z
x   y
NaN 1.0  1.0
2.0 2.0  2.0
3.0 NaN  3.0
4.0 4.0  4.0
NaN NaN -1.0
6.0 NaN -1.0
>>> pdf.fillna({'x': -1, 'y': -2, 'z': -5})
           z
x   y
NaN 1.0  1.0
2.0 2.0  2.0
3.0 NaN  3.0
4.0 4.0  4.0
NaN NaN -5.0
6.0 NaN -5.0

whereas:

>>> ks.from_pandas(pdf).fillna(-1)
             z
x    y
-1.0  1.0  1.0
 2.0  2.0  2.0
 3.0 -1.0  3.0
 4.0  4.0  4.0
-1.0 -1.0 -1.0
 6.0 -1.0 -1.0
>>> ks.from_pandas(pdf).fillna({'x': -1, 'y': -2, 'z': -5})
             z
x    y
-1.0  1.0  1.0
 2.0  2.0  2.0
 3.0 -2.0  3.0
 4.0  4.0  4.0
-1.0 -2.0 -5.0
 6.0 -2.0 -5.0

[codecov-io]

Codecov Report

Merging #1241 into master will decrease coverage by 1.27%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1241      +/-   ##
==========================================
- Coverage   95.05%   93.78%   -1.28%     
==========================================
  Files          35       35              
  Lines        7218     7219       +1     
==========================================
- Hits         6861     6770      -91     
- Misses        357      449      +92

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.84% <100%> (+0.11%)	⬆️
databricks/koalas/usage_logging/__init__.py	24.32% <0%> (-72.98%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50% <0%> (-50%)	⬇️
databricks/koalas/__init__.py	78.72% <0%> (-6.39%)	⬇️
databricks/conftest.py	94% <0%> (-4%)	⬇️
databricks/koalas/plot.py	94.28% <0%> (+0.95%)	⬆️

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[itholic]

LGTM 👍

[ueshin]

Thanks! I'd merge this for now since I found another issue related to fillna. I'll submit the PR soon.
Please feel free to leave comments if any.