Theme cannot add classes via preprocess webform date

In a theme you need to add css classes to the form element and add this code:

function foo_preprocess_webform_date(&$variables) {
  $variables['element']['year']['#attributes']['class'][] = 'foo-inline';
  $variables['element']['month']['#attributes']['class'][] = 'foo-inline';
  $variables['element']['day']['#attributes']['class'][] = 'foo-inline';
}

and this is just ignored because of:

/**
 * Theme a webform date element.
 */
function theme_webform_date($variables) {
  $element = $variables['element'];

  $element['year']['#attributes']['class'] = array('year');
  $element['month']['#attributes']['class'] = array('month');
  $element['day']['#attributes']['class'] = array('day');

This code need to be changed to:

/**
 * Theme a webform date element.
 */
function theme_webform_date($variables) {
  $element = $variables['element'];

  $element['year']['#attributes']['class'][] = 'year';
  $element['month']['#attributes']['class'][] = 'month';
  $element['day']['#attributes']['class'][] = 'day';