Fix Series.astype to work properly

[itholic]

This should fix #1806

>>> data = ks.Series(['a', 'b', 'c', None])
>>> sorted(data.astype(str).tolist())
['None', 'a', 'b', 'c']

For DataFrame.astype also works.

>>> kdf
   A     B     C
0  3  10.0     a
1  4  20.0     b
2  5  30.0     c
3  6  40.0     d
4  7  50.0  None

>>> sorted(kdf.astype(str).C.tolist())
['None', 'a', 'b', 'c', 'd']

[itholic]

Hmm... pandas>=1.1.1 seems work differently for None.

>>> pser = pd.Series(["hi", "hi ", " ", " \t", "", None], name="x")
>>> pser
0      hi
1     hi
2
3      \t
4
5    None
Name: x, dtype: object

>>> pser.astype(str)
0     hi
1    hi
2
3     \t
4
5    nan  # casted to `nan`
Name: x, dtype: object

Will report to pandas repo if this behavior is intended because it's not even written in the What's new in pandas 1.1.1

[itholic]

After pandas 1.1.4 is released and fixed related bug, I'll come back here to restore this.

Related issue: pandas-dev/pandas#36904 (comment)

[itholic]

I'd merge this now and revisit later for #1818 (comment)