Fix .loc to work properly when using 'slice'

[itholic]

Resolve #1158

>>> kdf
            max_speed  shield
cobra               1       2
viper               4       5
sidewinder          7       8

>>> kdf.loc['cobra':'viper', 'max_speed']
cobra    1
viper    4
Name: max_speed, dtype: int64

>>> kdf.to_pandas().loc['cobra':'viper', 'max_speed']
cobra    1
viper    4
Name: max_speed, dtype: int64

[codecov-io]

Codecov Report

Merging #1159 into master will increase coverage by 0.01%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master    #1159      +/-   ##
==========================================
+ Coverage   95.22%   95.24%   +0.01%     
==========================================
  Files          35       35              
  Lines        7058     7087      +29     
==========================================
+ Hits         6721     6750      +29     
  Misses        337      337

Impacted Files	Coverage Δ
databricks/koalas/series.py	96.54% <ø> (+0.1%)	⬆️
databricks/koalas/indexing.py	94.83% <96.77%> (+0.45%)	⬆️
databricks/koalas/testing/utils.py	78.51% <0%> (-0.75%)	⬇️
databricks/koalas/window.py	97.09% <0%> (-0.02%)	⬇️
databricks/koalas/base.py	96.34% <0%> (ø)	⬆️
databricks/koalas/groupby.py	91.6% <0%> (ø)	⬆️
databricks/koalas/missing/common.py	100% <0%> (ø)	⬆️
databricks/koalas/missing/indexes.py	100% <0%> (ø)	⬆️
databricks/koalas/frame.py	97.01% <0%> (ø)	⬆️
... and 4 more

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[itholic]

hmm.. i have no idea why codecov has failed although it seems that tests are covering all cases

[ueshin]

btw, we can remove line 236?

In addition, with a slice, Koalas works as a filter between the range.

[itholic]

Resolves #1173

[HyukjinKwon]

Looks fine otherwise.

[HyukjinKwon]

Thanks, merged.