Fix serialization of MutablePair. Also provide an interface for easy updating.

Author: Michael Armbrust <michael@databricks.com>

Closes #141 from marmbrus/mutablePair and squashes the following commits:

f5c4783 [Michael Armbrust] Change function name to update
8bfd973 [Michael Armbrust] Fix serialization of MutablePair.  Also provide an interface for easy updating.