Allow extended persistence context to be serialized.

Note EJB 3.1 specification section 4.2.1 titled Instance Passivation and Conversational State:
"
A container must not passivate a stateful session bean with an extended persistence context unless the
following conditions are met:[9]

• All the entities in the persistence context are serializable.
• The EntityManager is serializable.

[9] The container is not permitted to destroy a stateful session bean instance because it does not meet these requirements.
"