Fix Series.item to run a Spark job once

[itholic]

According to the comment #1502 (comment), fixed Series.item to run a Spark job not twice, once.

[codecov-commenter]

Codecov Report

Merging #1518 into master will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1518      +/-   ##
==========================================
+ Coverage   93.87%   94.02%   +0.15%     
==========================================
  Files          36       36              
  Lines        8469     8466       -3     
==========================================
+ Hits         7950     7960      +10     
+ Misses        519      506      -13     

Impacted Files	Coverage Δ
databricks/koalas/series.py	97.71% <100.00%> (-0.01%)	⬇️
databricks/koalas/frame.py	95.59% <0.00%> (+0.19%)	⬆️
databricks/koalas/generic.py	97.02% <0.00%> (+0.37%)	⬆️
databricks/koalas/utils.py	96.63% <0.00%> (+2.40%)	⬆️
databricks/koalas/__init__.py	92.30% <0.00%> (+5.76%)	⬆️

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[ueshin]

Otherwise, LGTM.