Fix transform to support multi-index columns.

[ueshin]

No description provided.

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Softagram Impact Report for pull/800 (head commit: a90966b)

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[codecov-io]

Codecov Report

Merging #800 into master will decrease coverage by 2.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #800      +/-   ##
==========================================
- Coverage    94.3%   92.03%   -2.27%     
==========================================
  Files          32       32              
  Lines        5828     5827       -1     
==========================================
- Hits         5496     5363     -133     
- Misses        332      464     +132

Impacted Files	Coverage Δ
databricks/koalas/frame.py	95.1% <100%> (-1.68%)	⬇️
databricks/koalas/usage_logging/__init__.py	23.14% <0%> (-74.08%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50% <0%> (-50%)	⬇️
databricks/koalas/__init__.py	77.5% <0%> (-7.5%)	⬇️
databricks/conftest.py	90.69% <0%> (-6.98%)	⬇️
databricks/koalas/namespace.py	84.81% <0%> (-1.33%)	⬇️
databricks/koalas/plot.py	93.34% <0%> (-0.96%)	⬇️
databricks/koalas/utils.py	97.24% <0%> (-0.69%)	⬇️
databricks/koalas/testing/utils.py	77.35% <0%> (-0.63%)	⬇️
... and 2 more

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[HyukjinKwon]

looks like we don't need this.

[ueshin]

Actually it is used at line 446.

koalas/databricks/koalas/frame.py

Lines 446 to 448 in a90966b

	@pandas_udf(returnType=as_spark_type(pser.dtype.type))
	def calculate_columns_axis(*cols):
	return getattr(pd.concat(cols, axis=1), name)(axis=axis, numeric_only=numeric_only)

[ueshin]

We can use F.pandas_udf there, but let me merge this for now.
Please feel free to submit the PR if needed.

[ueshin]

Thanks! merging.