rails3: fix issues with polymorphic type overrides

fixes:
read_attribute works slightly different in rails3, which surfaced a bug
with how polymorphic mappings swapped out the namespaced objects such as
QuizSubmission being replaced by Quizzes::QuizSubmission.

Change-Id: I91b6137e74d6015fd49bafa9f5c66a8f6e664ced
Reviewed-on: https://gerrit.instructure.com/31096
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>