This PR fixes an issue where PySpark broadcast variables caused NullPointerExceptions if KryoSerializer was used.  The fix is to register PythonBroadcast with Kryo so that it's deserialized with a KryoJavaSerializer.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #3831 from JoshRosen/SPARK-4882 and squashes the following commits:

0466c7a [Josh Rosen] Register PythonBroadcast with Kryo.
d5b409f [Josh Rosen] Enable registrationRequired, which would have caught this bug.
069d8a7 [Josh Rosen] Add failing test for SPARK-4882