`ImageField` and `GenericForeignKey` shouldn't connect `(pre|post)_init` signals to abstract senders
|Reported by:||charettes||Owned by:||nobody|
|Component:||Database layer (models, ORM)||Version:||master|
|Cc:||Triage Stage:||Ready for checkin|
|Has patch:||yes||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
Since (pre|post)_init is only sent by non abstract models (actually it is also sent when an abstract model is instantiated but one should not do that anyway) we should avoid polluting the associated signals registries with useless entries.
Attaching a patch with testcases making sure the changes didn't affect ImageField and GenericForeignKey inherited from abstract bases. The full test suite pass on Python 2.7 SQLite.