spec: fix fragile spec with dates

we recently added created_at/updated_at to the assignment_json response, and
this spec could run in such a way that they were off by 1 second.

test plan:
- specs should pass

Change-Id: I63e87c992e8f996b991d0d076e9a88419fe0e82e
Reviewed-on: https://gerrit.instructure.com/37224
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>