Fix header name and http method

For acknowledging a push queue message that was 202'd, made header and http method the same as shown one section below.

Ran code to test this and found
* There is no "Iron-Message" header in the push request from IronMQ
* POSTing to value of "Iron-Subscriber-Message-Url" returned 404 and retries kept going
* DELETEing to value of "Iron-Subscriber-Message-Url" returned 200 and the retries stopped