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 <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>