rails3: fix issues with polymorphic type overrides
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 <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Josh Simpson <bfb4987b134a00f73d881fff5113b1d5fa1876bc@instructure.com>
Product-Review: Derek DeVries <ff87a0776152510efd53bb1f4ea089dbbc90e730@instructure.com>
QA-Review: Derek DeVries <ff87a0776152510efd53bb1f4ea089dbbc90e730@instructure.com>

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