disallow write_attribute calls to create new attributes in rails 3
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 <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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