Fix on wrong error raise in DataFrame.fillna

[charlesdong1991]

koalas/databricks/koalas/frame.py

Line 3534 in 7ee2a8d

if (value is None) and (method is None):

seems this line won't be touched, and the reason it still passed tests is because it triggered the Series.fillna() error raise mechanism in the else below.

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• Co-change Alert: You modified frame.py. Often test_dataframe.py (koalas/tests) is modified at the same time.

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Codecov Report

Merging #844 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #844   +/-   ##
=======================================
  Coverage   94.31%   94.31%           
=======================================
  Files          32       32           
  Lines        5930     5930           
=======================================
  Hits         5593     5593           
  Misses        337      337

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.76% <100%> (+0.06%)	⬆️
databricks/koalas/series.py	95.07% <0%> (-0.17%)	⬇️

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[HyukjinKwon]

Thanks. Looks good but can we add a simple test @charlesdong1991?

[charlesdong1991]

i think there is a test already there:

koalas/databricks/koalas/tests/test_dataframe.py

Line 513 in 7ee2a8d

with self.assertRaisesRegex(ValueError,

but this triggered the error raising in Series.fillna. Do you still want a test? I could add one quickly @HyukjinKwon

[HyukjinKwon]

Ah, gotya. LGTM