fix failing specs on quiz_submissions for rails3

fixes:
- add an explicit after_save callback to save the assignment instead of before_save
- strip invalid characters from version_data input correctly
- fix error object formatting in assertions

Change-Id: Icdef876c63c4734e502d06a632fd9057a3bfd628
Reviewed-on: https://gerrit.instructure.com/29638
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>