fix submissions_controller_spec for rails3

Change-Id: I263bd8a98d6542c8fa469198ab2adec056276096
Reviewed-on: https://gerrit.instructure.com/30066
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Landon Wilkins <lwilkins@instructure.com>
QA-Review: Landon Wilkins <lwilkins@instructure.com>