fix quiz build_assignment for rails 3

setting self.assignment_id to nil also resets
self.assignment in rails 3 (but not in rails 2)

Change-Id: I679c0ebb8a26a6a4cb5ce5a69b688f094c75143d
Reviewed-on: https://gerrit.instructure.com/29646
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>