Added test to ensure post body is sent for PUT requests
Ajax bugfixes and enhancements

* Fixes inability to set Content-type header on methods other than POST
  - https://prototype.lighthouseapp.com/projects/8886/tickets/583-simulation-of-put-over-post-not-working-correctly
* Don't "emulate" standard HTTP verbs unless explicitly told to do so
  - https://prototype.lighthouseapp.com/projects/8886/tickets/289-allow-put-delete-and-other-http-methods-on-ajax-requests
  - https://prototype.lighthouseapp.com/projects/8886/tickets/1220-ajaxrequest-to-support-more-http-method-verbs-put-delete
  - https://rails.lighthouseapp.com/projects/8994/tickets/2289-_methodput-ignored-for-xhr-and-xml-requests
* When emulating HTTP verbs, use 'X-Http-Method-Override' header and _method param