Changed the behavior of EntityManager#getPartialReference to be read-only. No changes are ever
done to this entities. Changed UnitOfWork#computeChangeSet to never create a changeset for
fields that are partially omitted from a DQL or NativeQuery.
To check if an entity is read only use the new API:
if ($entityManager->getUnitOfWork()->isReadOnly($entity))
Now PHPUnit doesn't show warning about deprecation of assertType.
Also some refractoring from assertTrue($a instanceof $b) to assertInstanceOf.
Leading \ in namespaces is not required so I removed it from few assertions.
