[SPARK-5973] [PySpark] fix zip with two RDDs with AutoBatchedSerializer

Author: Davies Liu <davies@databricks.com>

Closes #4745 from davies/fix_zip and squashes the following commits:

2124b2c [Davies Liu] Update tests.py
b5c828f [Davies Liu] increase the number of records
c1e40fd [Davies Liu] fix zip with two RDDs with AutoBatchedSerializer