use assign_attributes instead of attributes= for rails 3 compatibility

Change-Id: I783618bc5457ea53a9d59ab36d3ccf7b7fde7b2e
Reviewed-on: https://gerrit.instructure.com/29595
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>