date_t and month_abbr (patch included)

date_t function don't work with abbr translations. For example, when calling date_t('Apr', 'month_abbr', 'en'), the returned string is ''.

This is because array $untranslated don't have the right keys.
A possible fix for month_abbr could be :

  case 'month_abbr':
    $untranslated = array_flip(array_map('_date_substr_3', date_month_names_untranslated()));
    break;
  case 'month_name':
    $untranslated = array_flip(date_month_names_untranslated());
    break;

This code need to add a new function :

function _date_substr_3($string) {
  return substr($string, 0, 3);
}