Model objects with deferred fields don't send pre_save or post_save signals to receivers with explicit sender

[Evan Heidtmann]

Because the objects returned by Model.objects.only(*fields) are of a special object type, and that type is used as the sender for the post_save signal send, these deferred objects never invoke signal handlers that registered with an explicit signal sender.

Something like this should reproduce:

`
# models.py
class MyModel(models.Model):

bigfield = models.TextField(null=True, blank=True)

# signals.py
@receiver.register(post_save, sender=models.MyModel)
def mymodel_post_save_handler(sender, kwargs):

print "handler called"

# repro code
m = MyModel.objects.create()
# (handler is called)
deferred_m = MyModel.objects.only('id').get(id=m.id)
deferred_m.bigfield = 'yo'
deferred_m.save()
# (handler should be called, but is not)
`

Happy to produce a proper test app/project if needed.

Evan