Added tests for accessing nullable ForeignKey after saving and fetching from...

>>> p.bestchild is None

True

# bestchild should still be None after saving.

>>> p.save()

>>> p.bestchild is None

True

# bestchild should still be None after fetching the object again.

>>> p = Parent.objects.get(name="Parent")

>>> p.bestchild is None

True

# Assigning None fails: Child.parent is null=False.

>>> c.parent = None

Traceback (most recent call last):