[PHPCR-98] allow replacing child documents via automatic merge()
Merge branch 'master' into PHPCR-98-handle-child-replacing
catch move single child by assignment. move replace child in multiple children to ChildrenTest and actually test replacing (fails)
fix handling of repository strategy for children collection
Merge pull request #253 from doctrine/PHPCR-98-handle-child-replacing

[PHPCR-98] and allow replacing child documents