[SPARK-7278] [PySpark] DateType should find datetime.datetime acceptable

DateType should not be restricted to `datetime.date` but accept `datetime.datetime` objects as well. Could someone with a little more insight verify this?

Author: ksonj <kson@siberie.de>

Closes #6057 from ksonj/dates and squashes the following commits:

68a158e [ksonj] DateType should find datetime.datetime acceptable too