Fix is_unique to respect the current Spark column

[ueshin]

No description provided.

[softagram-bot]

Softagram Impact Report for pull/981 (head commit: d556aa5)

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[codecov-io]

Codecov Report

Merging #981 into master will increase coverage by 1.49%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #981      +/-   ##
==========================================
+ Coverage   93.24%   94.74%   +1.49%     
==========================================
  Files          34       34              
  Lines        6487     6486       -1     
==========================================
+ Hits         6049     6145      +96     
+ Misses        438      341      -97

Impacted Files	Coverage Δ
databricks/koalas/series.py	96.32% <100%> (-0.01%)	⬇️
databricks/conftest.py	97.87% <0%> (+4.25%)	⬆️
databricks/koalas/__init__.py	85.36% <0%> (+7.31%)	⬆️
databricks/koalas/usage_logging/usage_logger.py	100% <0%> (+50%)	⬆️
databricks/koalas/usage_logging/__init__.py	97.27% <0%> (+72.72%)	⬆️

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[HyukjinKwon]

Ah, nice. LGTM