GenericForeignKey fails if initialized to None during model init

[Sam Bull]

Supposing you have a model with a GenericForeignKey...

class Example(models.Model):
    name = models.CharField()
    ct = models.ForeignKey(ContentType, null=True, blank=True)
    obj_id = models.PositiveIntegerField(null=True, blank=True)
    obj = GenericForeignKey()

If you initialize like this, everything is fine:

ex = Example(name='foobar')

But if you do this...

ex = Example(name='foobar', obj=None)

You get an exciting error:
Impossible arguments to GFK.get_content_type!

It looks like the GFK tries to initialize the content_type and object_id fields if its attribute is one of the ones passed to the init. In fact, it should follow the example of get and only initialize those fields if the argument passed to it is non-None.

[Sam Bull]

GFK-init patch