Fix: select_related on proxy models fails: get_field_by_name result interpretation is incorrect

While using select_related on a proxy TranslatableModel, I found out that the query generated is incorrect, because the interpretation of get_field_by_name result tuple assumes that if a model is returned, it means it is a relationship. This is wrong in the case of proxy model. This fix inteprets all cases correctly, and raises an error if it meets a regular, non-relational field.