don't overwrite form factory if already set
Merge branch 'patch-1' of git://github.com/radnan/zf2 into hotfix/3735
[#3735] Added unit test

- Test demonstrates that factory composed by object implementing
  FormFactoryAwareInterface is not overwritten, but *is* injected with
  FormElementManager instance.
Merge branch 'hotfix/3735'

Close #3735