[SPARK-15244] [PYTHON] Type of column name created with createDataFrame is not consistent.

## What changes were proposed in this pull request?

**createDataFrame** returns inconsistent types for column names.
```python
>>> from pyspark.sql.types import StructType, StructField, StringType
>>> schema = StructType([StructField(u"col", StringType())])
>>> df1 = spark.createDataFrame([("a",)], schema)
>>> df1.columns # "col" is str
['col']
>>> df2 = spark.createDataFrame([("a",)], [u"col"])
>>> df2.columns # "col" is unicode
[u'col']
```

The reason is only **StructField** has the following code.
```
if not isinstance(name, str):
    name = name.encode('utf-8')
```
This PR adds the same logic into **createDataFrame** for consistency.
```
if isinstance(schema, list):
    schema = [x.encode('utf-8') if not isinstance(x, str) else x for x in schema]
```

## How was this patch tested?

Pass the Jenkins test (with new python doctest)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #13097 from dongjoon-hyun/SPARK-15244.