Fix Series.unstack to support non-numeric type and keep the names

[itholic]

According to #1501 (comment) and #1501 (comment), Fix Series.unstack() to support non-numeric type and keep the names of index and columns.

>>> kser = ks.Series(list('abcd'), index=pd.MultiIndex.from_product([['one', 'two'], ['a', 'b']], names=["A", "B"]))
>>> kser
A    B
one  a    a
     b    b
two  a    c
     b    d
Name: 0, dtype: object

>>> kser.unstack()
B    a  b
A
two  c  d
one  a  b

[codecov-commenter]

Codecov Report

Merging #1527 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1527   +/-   ##
=======================================
  Coverage   94.14%   94.14%           
=======================================
  Files          37       37           
  Lines        8487     8491    +4     
=======================================
+ Hits         7990     7994    +4     
  Misses        497      497           

Impacted Files	Coverage Δ
databricks/koalas/series.py	97.76% <100.00%> (+0.01%)	⬆️

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[HyukjinKwon]

LGTM otherwise.