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.
well it is kind of needed, so I do not see the advantage of using setter injection .. well aside from theoretically less need to mock stuff in some unit tests I guess ..