fix serialized attributes for rails 3

in particular, prevent it from initializing when nil,
and prevent it from overwriting the value with the serialized
value (this causes problems in specs when mocking for attributes
such as Attachment.scribd_doc)

Change-Id: I4c37d9222e51a85da692dc351d515e0f326f435b
Reviewed-on: https://gerrit.instructure.com/28899
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>