Fixed #22210 -- Saving model instances to non-related fields.

Previously, saving a model instance to a non-related field (in
particular a FloatField) would silently convert the model to an Integer
(the pk) and save it. This is undesirable behaviour, and likely to cause
confusion so the validatio has been hardened.

Thanks to @PirosB3 for the patch and @jarshwah for the review.