belongs_to polymorphic association assignments update the foreign_id and foreign_type fields regardless of whether the record being assigned is new or not.

fixes the following scenarios:
* I have validates_inclusion_of on the type field for a polymorphic belongs_to association.  I assign a new record to the model's polymorphic relationship of the proper type.  validation fails because the type field has not been updated.

* I replace the value for a ppolymorphic association to a new record of another class.  The type field still says its the previous class, and the id field points to the previous record as well.