Adjust data column names before attaching default index.

[ueshin]

Adjust data column names before attaching default index; otherwise reset_index() could fail when the DataFrame or Series is converted from Index.

>>> kdf = ks.Index([10, 20, 30]).to_frame()
>>> kdf
     0
10  10
20  20
30  30
>>> kdf.reset_index()
Traceback (most recent call last):
...
AssertionError: Index columns should not appear in columns of the Spark DataFrame. Avoid index column names [re.compile('__index_level_[0-9]+__')].

or

>>> kser = ks.Index([10, 20, 30]).to_series()
>>> kser
10    10
20    20
30    30
dtype: int64
>>> kser.reset_index(drop=True)
Traceback (most recent call last):
...
AssertionError: Index columns should not appear in columns of the Spark DataFrame. Avoid index column names [re.compile('__index_level_[0-9]+__')].

[codecov-io]

Codecov Report

Merging #1947 (c761493) into master (138c7b8) will decrease coverage by 2.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1947      +/-   ##
==========================================
- Coverage   94.64%   92.10%   -2.55%     
==========================================
  Files          49       49              
  Lines       10818    10711     -107     
==========================================
- Hits        10239     9865     -374     
- Misses        579      846     +267     

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.09% <100.00%> (-0.67%)	⬇️
databricks/koalas/series.py	96.91% <100.00%> (-0.13%)	⬇️
...bricks/koalas/tests/plot/test_frame_plot_plotly.py	26.58% <0.00%> (-73.42%)	⬇️
databricks/koalas/usage_logging/__init__.py	25.66% <0.00%> (-66.65%)	⬇️
...ricks/koalas/tests/plot/test_series_plot_plotly.py	37.50% <0.00%> (-58.80%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	47.82% <0.00%> (-52.18%)	⬇️
databricks/koalas/__init__.py	79.68% <0.00%> (-10.94%)	⬇️
databricks/koalas/accessors.py	86.43% <0.00%> (-7.04%)	⬇️
databricks/conftest.py	93.22% <0.00%> (-6.78%)	⬇️
databricks/koalas/generic.py	88.18% <0.00%> (-4.74%)	⬇️
... and 20 more

---

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[HyukjinKwon]

Thanks Takuya. I think this looks fine to me.

[ueshin]

Thanks! merging.