[SPARK-1687] [PySpark] fix unit tests related to pickable namedtuple

serializer is imported multiple times during doctests, so it's better to make _hijack_namedtuple() safe to be called multiple times.

Author: Davies Liu <davies.liu@gmail.com>

Closes #1771 from davies/fix and squashes the following commits:

1a9e336 [Davies Liu] fix unit tests
Merge pull request #3 from apache/master

[SPARK-1687] [PySpark] fix unit tests related to pickable namedtuple