Fix column name as a tuple in multi column index

[HyukjinKwon]

import pandas as pd
import databricks.koalas as ks
column_index = pd.MultiIndex.from_tuples([('a', 'x'), ('a', 'y'), ('b', 'z')])
pdf = pd.DataFrame([[1, 2, 3], [4, 5, 6], [7, 8, 9]], columns=column_index)
pdf.set_index(('a', 'x'), append=True, inplace=True)
ks.from_pandas(pdf)

Before:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../koalas/databricks/koalas/namespace.py", line 70, in from_pandas
    return DataFrame(pobj)
  File "/.../koalas/databricks/koalas/frame.py", line 380, in __init__
    super(DataFrame, self).__init__(_InternalFrame.from_pandas(pdf))
  File "/.../koalas/databricks/koalas/internal.py", line 771, in from_pandas
    column_index_names=column_index_names)
  File "/.../koalas/databricks/koalas/internal.py", line 431, in __init__
    for index_field, index_name in index_map), index_map
AssertionError: [('__index_level_0__', None), (('a', 'x'), ('a', 'x'))]

After:

          a  b
          y  z
  (a, x)
0 1       2  3
1 4       5  6
2 7       8  9

[codecov-io]

Codecov Report

Merging #1191 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1191   +/-   ##
=======================================
  Coverage   95.22%   95.22%           
=======================================
  Files          35       35           
  Lines        7141     7141           
=======================================
  Hits         6800     6800           
  Misses        341      341

Impacted Files	Coverage Δ
databricks/koalas/internal.py	95.72% <100%> (ø)	⬆️

---

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[softagram-bot]

Softagram Impact Report for pull/1191 (head commit: b915c4a)

⚠️ Copy paste found

ℹ️ test_indexes.py: Copy paste fragment on line 32 shared with ../test_numpy_compat.py:

    def pdf(self):
        return pd.DataFrame({
            'a': [1, 2, 3, 4, 5, 6, 7, 8, 9],
            'b': [4, 5, 6, 3, 2, 1, 0, 0, 0],
        }, index=[0, 1, 3, 5, 6, 8, 9, 9, 9])...(truncated 101 chars)

Now that you are on the file, it would be easier to pay back some tech. debt.

⭐ Change Overview

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💡 Insights

• Co-change Alert: You modified test_indexes.py. Often indexes.py (databricks/koalas) is modified at the same time.

📄 Full report

• Permalink: Full report for pull/1191

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[ueshin]

LGTM.

[ueshin]

Thanks! merging.