fix assignments groups controller spec for rails 3

Change-Id: I738b85df1b21603d0fbfda6017ad7a360f9722a9
Reviewed-on: https://gerrit.instructure.com/29775
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>