[SPARK-8535] [PYSPARK] PySpark : Can't create DataFrame from Pandas dataframe with no explicit column name

Because implicit name of `pandas.columns` are Int, but `StructField` json expect `String`.
So I think `pandas.columns` are should be convert to `String`.

### issue

* [SPARK-8535 PySpark : Can't create DataFrame from Pandas dataframe with no explicit column name](https://issues.apache.org/jira/browse/SPARK-8535)

Author: x1- <viva008@gmail.com>

Closes #7124 from x1-/SPARK-8535 and squashes the following commits:

d68fd38 [x1-] modify unit-test using pandas.
ea1897d [x1-] For implicit name of pandas.columns are Int, so should be convert to String.