Pre-loading select list options by AJAX does not work in D7

The Drupal 7 version of Webform 3.x doesn't properly preload select list options. This is because we're both returning a JSON object and then using the AJAX-delivery mechanism in Drupal 7, when it should be one-or-the-other. So this patch sticks with the good-old drupal_json_output() function and gets our loading of options working properly again.