Make reset_index disallow the same name but allow it when drop=True.

[ueshin]

pandas' DataFrame.reset_index() raises an error if the index name is the same as one of columns but allow it when drop=True.

>>> import pandas as pd
>>> import numpy as np
>>> pdf = pd.DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]}, index=np.random.rand(3))
>>> pdf.index.name = "a"
>>> pdf.reset_index()
Traceback (most recent call last):
...
ValueError: cannot insert a, already exists
>>> pdf.reset_index(drop=True)
   a  b
0  1  4
1  2  5
2  3  6

whereas Koalas raises another error for both cases:

>>> ks.from_pandas(pdf).reset_index()
Traceback (most recent call last):
...
pyspark.sql.utils.AnalysisException: "Reference 'a' is ambiguous, could be: a, a.;"

>>> ks.from_pandas(pdf).reset_index(drop=True)
...
pyspark.sql.utils.AnalysisException: "Reference 'a' is ambiguous, could be: a, a.;"

[itholic]

LGTM.

[ueshin]

Thanks! merging.