fix DayOfWeekField::validate() for multiple days with week of month, like MON#1,MON#3

See https://github.com/mtdowling/cron-expression/issues/70.

This fix can, if desired at all, most certainly be done in a more elegant way using regex only (if not getting too complicated itself then), but it's at least a workaround for now to prevent the Exception.

ps @mtdowling thanks for your great work both on Guzzle and this library.