fix crocodoc sessions controller for rails 3 specs

Change-Id: Ie0478d907e6cf8a75f0303f8d7b8d038d960d752
Reviewed-on: https://gerrit.instructure.com/29876
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>