Theme cannot add classes via preprocess to webform_advanced_submit_limit_form

Bugfix [webform.pages.inc]

/**
 * Theme the submit limit fieldset on the webform node form.
 */
function theme_webform_advanced_submit_limit_form($variables) {
  $form = $variables['form'];
  //$form['submit_limit']['#attributes']['class'] = array('webform-set-active');
  //$form['submit_interval']['#attributes']['class'] = array('webform-set-active');
  $form['submit_limit']['#attributes']['class'][] = 'webform-set-active';
  $form['submit_interval']['#attributes']['class'][] = 'webform-set-active';