Add some missing tests for namespace

[itholic]

I found some untested codes in namespace.py like:

so i added tests for cover them.

I would appreciate it if someone available.

[HyukjinKwon]

Looks good if tests pass

[codecov-io]

Codecov Report

Merging #711 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #711      +/-   ##
==========================================
+ Coverage   94.01%   94.17%   +0.16%     
==========================================
  Files          32       32              
  Lines        5548     5548              
==========================================
+ Hits         5216     5225       +9     
+ Misses        332      323       -9

Impacted Files	Coverage Δ
databricks/koalas/namespace.py	93.72% <0%> (+3.52%)	⬆️

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[itholic]

@HyukjinKwon I've fixed them. Thanks for comment !