Fixed ks.Index.to_series() to work properly with name paramter

[itholic]

The existing ks.Index.to_series() work not properly with name parameter
(Although Its test was passed !)

>>> kidx = ks.Index([1, 2, 3])
>>> kidx.to_series(name="Koalas")
1    1
2    2
3    3
Name: 0, dtype: int64  # It should have renamed to 'Koalas'

This PR addressed it.

[itholic]

The existing test code below has been passing even though the name was not changed properly.

self.assert_eq(kidx.to_series(name="a"), pidx.to_series(name="a"))

>>> kidx.to_series(name="a")
0    0
1    1
3    3
5    5
6    6
8    8
9    9
9    9
9    9
Name: 0, dtype: int64

>>> pidx.to_series(name="a")
0    0
1    1
3    3
5    5
6    6
8    8
9    9
9    9
9    9
Name: a, dtype: int64

For now, I addressed it with repr, but maybe I think we'd better to find the better solution ?

[ueshin]

What if ks.Index([1, 2, 3], name=‘a’).to_series() ?

[itholic]

What if ks.Index([1, 2, 3], name=‘a’).to_series() ?

Oh, I think we should manage it, too.

Just addressed it, Thanks! :)

FYI:

>>> pd.Index([1, 2, 3], name="a").to_series()  # example of pandas
a
1    1
2    2
3    3
Name: a, dtype: int64

>>> ks.Index([1, 2, 3], name="a").to_series()  # example of Koalas
a
1    1
2    2
3    3
Name: 0, dtype: int64  # TODO: the name of Series also should be "a" which is the same name with index

[itholic]

@ueshin

Anyway, could you help me to solve this mypy failure when you available ?
Since I'm not familiar with mypy that much, couldn't find the solution yet. 😿

[ueshin]

Otherwise, LGTM.