Extra unit tests for check_delete_headers

A few extra tests to verify check_delete_headers in
constraints.py. A little duplication of coverage of existing
proxy/controllers/test_obj.py:TestObjController.test_POST_delete_at
but these tests call the recently refactored function directly,
and also add tests for X-Delete-After taking precedence over
X-Delete-At.

Change-Id: I129cef15a6feac8a60fd4efbb3535d93f0eaab36
Merge "Extra unit tests for check_delete_headers"