(PUP-2505) Implement DELETE verb for HTTP routing

Duplicates:
   PUP-2483 Deleting a certificate does not work with the API
   PUP-2516 Delete a clients certificate with HTTP API no longer working

HTTP API requests using the DELETE method are failing with "Server Error:
undefined method `each' for nil:NilClass"...

e.g.
 curl -k -X DELETE https://${PMASTER}:8140/production/certificate_status/$AGENT

Traced problem back to missing specification for :DELETE in @method_handlers
(PUP-2505) Add method to specify delete handlers

Amending pull request per review comment.  Adding handler for delete
method.
(PUP-2505) Add test for delete handling
(PUP-2505) Reject requests with unknown HTTP verbs

When we forgot to handle DELETE the error showed up as a 500 because the
code tried to call a method on nil. The correct response is a 405, since
the server does not handle that method. Now that GET, POST, PUT, HEAD,
and DELETE are all covered, it should be safe to respond with 405 in all
other cases.
Merge remote-tracking branch 'zaphod42/issue/stable/pup-2505-delete-not-respected' into stable

* zaphod42/issue/stable/pup-2505-delete-not-respected:
  (PUP-2505) Reject requests with unknown HTTP verbs
  (PUP-2505) Add test for delete handling
  (PUP-2505) Add method to specify delete handlers
  (PUP-2505) Implement DELETE verb for HTTP routing