Allow customization of ajax request in devise authenticator.

[kbullaughey]

Allow makeRequest to be overridden to provide a second parameter that gets merge into .ajax() config argument. Here's an example of how this could be used to have devise with a json body instead of request parameters:

export default DeviseAuthenticator.extend({
  makeRequest: function(data) {
    if (typeof(data) === "object") {
      data = JSON.stringify(data);
    }
    return this._super(data, {contentType: 'application/json'});
  }
});