disallow write_attribute calls to create new attributes in rails 3

just like in rails 4

fixes CNVS-15830

Change-Id: Id27524b124a24110729c17497c45741063a4d7a2
Reviewed-on: https://gerrit.instructure.com/41790
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>