JpaPersistService.begin() throws exception if a Unit of Work is already opened

[jccarrillo]

The implementation of UnitOfWork.begin() within JpaPersistService does not comply with the definition specified in the javadoc.

"If there is already one open, the invocation will do nothing. In this way, you can define arbitrary units-of-work that nest within one another safely."

The above is not true, the implementation throws an exception if "one" is already opened.

[skuzzle]

Duplicate of #947. Thanks for the fix. I hope it will be accepted.

Edit: Misread it, the PersistServices still faces the same issue.