fix rails3 spec that checks missing referer value

Change-Id: I6c36fd2b5be9cad941f60f852b0124ca8720dd46
Reviewed-on: https://gerrit.instructure.com/29807
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>