make PHPCR ODM 5.6 compatible

fixing the bug with creating new class instances

fixes https://github.com/doctrine/phpcr-odm/issues/534

however we need to wait for Doctrine to release a stable version but imho we should already merge this.
I just don't see why somebody would do that in first place. If a different instantiator is needed, then I see a use-case for replacing the entire classmetadata instance, no?