rails3: fix specs for assignment override student
rails3: fix specs for assignment override student

fixes:
in rails3 setting the assignment_id in the before_validation :default_values
filter also assigns assignment object to match. this makes it the validation
never fail in rails3

Change-Id: I07c303945484953800a71e9b2eba90cad617f7e3
Reviewed-on: https://gerrit.instructure.com/30756
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Product-Review: Derek DeVries <ff87a0776152510efd53bb1f4ea089dbbc90e730@instructure.com>
QA-Review: Derek DeVries <ff87a0776152510efd53bb1f4ea089dbbc90e730@instructure.com>

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