## 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.
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