If you override ajaxSettings and change the method/type, it was
getting ignored in _ajax(). This uses the ajaxSettings method rather
than the one passed in, giving users the ability to talk to
non-traditional APIs.

You can specify a callback on success/error in your ajaxSettings so you
can do something like:

var mySuccess = function(data, status, jqXHR) {
  console.log("success");
}

var myError = function(jqXHR, status, error) {
  console.warn("error");
}

var ajaxSettings = function(url, method) {
  return {
    url: url,
    type: method,
    successCallback: mySuccess,
    errorCallback: myError
  };
}

If your API is a bit goofy you can override the ajaxSettings in your
RESTAdapter so you can do things like:

--> POST URL-Encoded, receive JSON back:

  myModel.adapter = RESTAdapter.extend({
    ajaxSettings: function(url) {
      type: "POST",
      contentType: "application/x-www-form-urlencoded",
      dataType: "json"
      // ... snip other properties
    }
  });

--> PUT JSON encoded content, receive text back

  myModel.adapter = RESTAdapter.extend({
    ajaxSettings: function(url) {
      type: "PUT",
      contentType: "json",
      dataType: "text"
      // ... snip other properties
    }
  });

This was causing previously working calls to fail because now our ajax
calls have the Content-Type: header being added, making the server
return a 400 Bad Request error.

# Conflicts:
#	ember-model.js
#	packages/ember-model/lib/attr.js
#	packages/ember-model/lib/belongs_to.js
#	packages/ember-model/lib/has_many.js
#	packages/ember-model/lib/rest_adapter.js

# Conflicts:
#	ember-model.js