Use parent's primary key when model is derived via multitable inheritance

Currently, the following code will fail:

    class ParentModel (Model):
        name1 = CharField(max_length=100)

    class ChildModel (ParentModel):
        """Derived from ParentModel"""
        name2 = CharField(max_length=100)

    class DerivedModelSerializer (ModelSerializer):
        class Meta:
            model = ChildModel

    serializer = DerivedModelSerializer(data={
        'name1': 'abc', 'name2': 'xyz'
    })
    assert serializer.is_valid()

DRF will expect parentmodel_ptr to be a field on the serializer. This should not be expected.  The given patch fixes this.