fix lti grade passback spec for rails 3

Change-Id: Id3659d3673ec46fa8658b1c8ca24b9120c0bb918
Reviewed-on: https://gerrit.instructure.com/30698
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>