Allow multi-index column names for inferring return type schema with names.

[ueshin]

Allow multi-index column names for inferring return type schema with names.

• Before

>>> pdf = pd.DataFrame({("x", "a"): [1, 2, 3], ("y", "b"): [3, 4, 5]})
>>> def func() -> ks.DataFrame[zip(pdf.columns, pdf.dtypes)]:
...     pass
>>> infer_return_type(func).tpe
Traceback (most recent call last):
...
TypeError: not all arguments converted during string formatting

• After

>>> pdf = pd.DataFrame({("x", "a"): [1, 2, 3], ("y", "b"): [3, 4, 5]})
>>> def func() -> ks.DataFrame[zip(pdf.columns, pdf.dtypes)]:
...     pass
>>> infer_return_type(func).tpe
StructType(List(StructField((x, a),LongType,true),StructField((y, b),LongType,true)))

[codecov-io]

Codecov Report

Merging #2117 (db4c122) into master (deccf33) will decrease coverage by 0.72%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2117      +/-   ##
==========================================
- Coverage   95.36%   94.64%   -0.73%     
==========================================
  Files          60       60              
  Lines       13544    13545       +1     
==========================================
- Hits        12916    12819      -97     
- Misses        628      726      +98     

Impacted Files	Coverage Δ
databricks/koalas/typedef/typehints.py	94.56% <100.00%> (+0.02%)	⬆️
databricks/koalas/usage_logging/__init__.py	27.27% <0.00%> (-65.29%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	47.82% <0.00%> (-52.18%)	⬇️
databricks/conftest.py	93.75% <0.00%> (-6.25%)	⬇️
databricks/koalas/__init__.py	88.15% <0.00%> (-3.95%)	⬇️

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[itholic]

LGTM

[ueshin]

Thanks! merging.