[SPARK-1630] Turn Null of Java/Scala into None of Python

During serializing PythonRDD, it will cause an NPE if there null in it. This patch will handle it as None of Python.

This PR is based on #554, thanks to @kalpit. 
It's header of var-length field, it's better to keep this header has fixed length, or you will need to deal with special var-length encoding.