spec: fix assignment_spec for rails 3

Change-Id: I64284315ef42505595e8100405258ecb08ea7a1b
Reviewed-on: https://gerrit.instructure.com/31693
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>