Add support scalar values as row_sel for loc indexer.

[ueshin]

Resolves #383.

[codecov-io]

Codecov Report

Merging #1172 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1172      +/-   ##
==========================================
+ Coverage   95.26%   95.28%   +0.01%     
==========================================
  Files          35       35              
  Lines        7061     7084      +23     
==========================================
+ Hits         6727     6750      +23     
  Misses        334      334

Impacted Files	Coverage Δ
databricks/koalas/series.py	96.46% <100%> (-0.01%)	⬇️
databricks/koalas/indexing.py	95.98% <100%> (+0.33%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 72f95bc...0ace2fd. Read the comment docs.