 If you want to not use `iron-ajax` at all, you can cancel the
event and do your own custom submission:

  Example of modifying the request, but still using the build-in form submission:

    form.addEventListener('iron-form-presubmit', function() {
      this.request.method = 'put';
      this.request.params['extraParam'] = 'someValue';
    });

  Example of bypassing the build-in form submission:

    form.addEventListener('iron-form-presubmit', function(event) {
      event.preventDefault();
      var fi