Fix index operator against Series and Frame to use iloc conditionally

[HyukjinKwon]

Looks pandas conditionally uses iloc approach when the input is int against index operators in Series and Frame.

import databricks.koalas as ks
import pandas as pd
import numpy as np

dates = pd.date_range('20130101', periods=6)
pdf = pd.DataFrame(np.random.randn(6,4), index=dates, columns=list('ABCD'))
kdf = ks.from_pandas(pdf)
kdf[0:3]

                   A         B         C         D
2013-01-01  2.143975 -2.069608 -0.013988 -0.311598
2013-01-02 -0.448577  0.076479 -0.356269  0.965872
2013-01-03  0.203477  0.738288 -1.453629 -0.146019

Resolves #287

[codecov-io]

Codecov Report

Merging #1336 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1336      +/-   ##
==========================================
+ Coverage   95.15%   95.17%   +0.01%     
==========================================
  Files          34       34              
  Lines        7472     7476       +4     
==========================================
+ Hits         7110     7115       +5     
+ Misses        362      361       -1     

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.71% <100.00%> (+<0.01%)	⬆️
databricks/koalas/series.py	96.74% <100.00%> (+<0.01%)	⬆️
databricks/koalas/indexing.py	95.28% <0.00%> (+0.24%)	⬆️

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[itholic]

LGTM

[ueshin]

LGTM.
btw, the PR description does not make sense? maybe need to add some examples accessing by index operator?

[HyukjinKwon]

oops, sure. I missed the line from the issue.

[HyukjinKwon]

Merged! thanks, @ueshin and @itholic