remove AR serialization overrides

fixes CNVS-15784

restores default behavior of a nil column returning a blank object,
rather than nil

had to adjust turnitin code that expected a nil rather than blank,
and a few specs

test plan:
 * regression test turnitin integration

Change-Id: I62968cabdba15f2e59ebdfed9b946781a1b0561c
Reviewed-on: https://gerrit.instructure.com/41647
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>