Fixed flaky test for instructor command openended_post.

The submission time is serialized into json and sent to xqueue. Previously the
whole json body argument to send_to_queue() was being matched. Sometimes the
time calculated in the test would differ slightly from the time calculated in
send_to_grader() and the test would fail. This fix tests individual fields of
the body argument to mock_send_to_queue() instead.
Merge pull request #2291 from edx/usman/fix-openended-post-command-tests

Fixed flaky test for instructor command openended_post.