Add missing tests for sql

[itholic]

As you can see in the following screenshot,

There were some missing sql test using Series, so I added it to Doc test.

could someone check this change maybe if available?

Thanks.

[HyukjinKwon]

Looks good otherwise

[codecov-io]

Codecov Report

Merging #733 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #733      +/-   ##
==========================================
+ Coverage    94.3%   94.32%   +0.01%     
==========================================
  Files          32       32              
  Lines        5570     5570              
==========================================
+ Hits         5253     5254       +1     
+ Misses        317      316       -1

Impacted Files	Coverage Δ
databricks/koalas/sql.py	95.83% <ø> (+1.04%)	⬆️

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