don't include values for defaulted columns on insert

test plan:
 * in script/console, create a new user
 * only set the name
 * save it
 * the insert sql should have a limited number of attributes
   (name, uuid, sortable_name, created_at, updated_at, workflow_state)

Change-Id: I0de90558efd288f47ce68bc5491affd365be18bb
Reviewed-on: https://gerrit.instructure.com/36513
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>