Function mb_substr should replace substr

in date_api.module (line 175) we have this:

function date_week_days_abbr($required = FALSE, $refresh = TRUE, $length = 3) {
  if ($refresh || empty($weekdays)) {
    $weekdays = array();
    foreach (date_week_days($refresh) as $key => $day) {
      $weekdays[$key] = substr($day, 0, $length);
    }
  }
  $none = array('' => '');
  return !$required ? $none + $weekdays : $weekdays;
}

This code should use mb_substr instead of substr because it is using utf8 strings.
For example, with a three lenght, 'Wednesday' and 'Saturday' in spanish are 'Miércoles' and 'Sábado', which will end in 'Mi�' and 'Sá' instead of 'Mié' and 'Sab'.