fix rails 3 building of new has_one objects where the foreign key is the primary key

Change-Id: I356dda316ca47b24012ee830aa133caa085e5c71
Reviewed-on: https://gerrit.instructure.com/30704
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>