ensure assignments API uses correct date time format part1

refs CNVS-10087

test plan
 - assignments PUT and CREATE request will not
   change, but will create error_report when dates
   are not is08601 formatted dates

Change-Id: I83e3621c0604cf91e180f5e59bea2d6e6a164c61
Reviewed-on: https://gerrit.instructure.com/32328
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Reviewed-by: Anthus Williams <awilliams@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>