Fix Series.__getitem__ to work properly

[itholic]

Resolves #933

>>> kser = ks.Series([10, 20, 15, 30, 45], ['A', 'A', 'B', 'C', 'D'])
>>> kser['A']
A    10
A    20
Name: 0, dtype: int64
>>>
>>> kser['B']
15

For MultiIndex

>>> midx = pd.MultiIndex([['lama', 'cow', 'falcon'],
...                       ['speed', 'weight', 'length']],
...                      [[0, 0, 0, 1, 1, 1, 2, 2, 2],
...                       [0, 1, 2, 0, 1, 2, 0, 1, 2]])
>>> kser = ks.Series([45, 200, 1.2, 30, 250, 1.5, 320, 1, 0.3], index=midx)
>>> pser = kser.to_pandas()

>>> kser['lama']
speed      45.0
weight    200.0
length      1.2
Name: 0, dtype: float64

>>> pser['lama']
speed      45.0
weight    200.0
length      1.2
Name: 0, dtype: float64

[codecov-io]

Codecov Report

Merging #934 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #934      +/-   ##
==========================================
+ Coverage   94.47%   94.49%   +0.01%     
==========================================
  Files          34       34              
  Lines        6392     6425      +33     
==========================================
+ Hits         6039     6071      +32     
- Misses        353      354       +1

Impacted Files	Coverage Δ
databricks/koalas/series.py	95.86% <100%> (+0.25%)	⬆️
databricks/koalas/utils.py	97.97% <100%> (+0.02%)	⬆️
databricks/koalas/indexes.py	96.42% <0%> (-0.96%)	⬇️
databricks/koalas/missing/indexes.py	100% <0%> (ø)	⬆️
databricks/koalas/base.py	94.83% <0%> (+0.03%)	⬆️

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[ueshin]

Seems like the examples for MultiIndex are different between Koalas and pandas?

[itholic]

@ueshin oops, i think it was just my mistake

they actually work same.

>>> midx = pd.MultiIndex([['lama', 'cow', 'falcon'],
...                       ['speed', 'weight', 'length']],
...                      [[0, 0, 0, 1, 1, 1, 2, 2, 2],
...                       [0, 1, 2, 0, 1, 2, 0, 1, 2]])
>>> kser = ks.Series([45, 200, 1.2, 30, 250, 1.5, 320, 1, 0.3], index=midx)
>>> pser = kser.to_pandas()

>>> kser['lama']
speed      45.0
weight    200.0
length      1.2
Name: 0, dtype: float64

>>> pser['lama']
speed      45.0
weight    200.0
length      1.2
Name: 0, dtype: float64

(fix existing example in PR description also)

[HyukjinKwon]

Looks good otherwise.

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