use assert_status instead of checking response.status directly

since it's not a string in rails 3

Change-Id: Ib1859a3ca241bd64ca0c43b4ccfec94ba801eab6
Reviewed-on: https://gerrit.instructure.com/29780
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>