fix discussion topics controller spec for rails 3

Change-Id: Ic41d5a9fc528fab24cd54a23320b1b3a0218f87f
Reviewed-on: https://gerrit.instructure.com/29880
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>