Fix Series.clip not to create a new DataFrame.

[ueshin]

Currently Series.clip creates a new DataFrame, so assigning back to the anchor DataFrame can't be executed without the option "compute.ops_on_diff_frames":

>>> kdf = ks.DataFrame({"a": [1, 2, 6, 4, 4, 6, 4, 3, 7]})
>>> kdf['clip'] = kdf.a.clip(3, 6)
Traceback (most recent call last):
...
ValueError: Cannot combine the series or dataframe because it comes from a different dataframe. In order to allow this operation, enable 'compute.ops_on_diff_frames' option.

[codecov-commenter]

Codecov Report

Merging #1525 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1525   +/-   ##
=======================================
  Coverage   94.02%   94.02%           
=======================================
  Files          36       36           
  Lines        8479     8480    +1     
=======================================
+ Hits         7972     7973    +1     
  Misses        507      507           

Impacted Files	Coverage Δ
databricks/koalas/frame.py	95.56% <100.00%> (-0.03%)	⬇️
databricks/koalas/series.py	97.74% <100.00%> (+0.02%)	⬆️

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