[1.6.x] Fixed related model lookup regression

It has been possible to use models of wrong type in related field
lookups. For example pigs__in=[a_duck] has worked. Changes to
ForeignObject broke that.

It might be a good idea to restrict the model types usable in lookups.
This should be done intentionally, not accidentally and without any
consideration for deprecation path.

Backpatch of 7cca8d56d28e321ffc395c92f82d97adaa0dcf94 from master.