fix assignment_override_student for rails 3

Change-Id: I4362e421bbc2a23c5afdadcc5658abe17e8a1402
Reviewed-on: https://gerrit.instructure.com/28693
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>