A PRIVMSG received for "#MyChan" and #mychan" must go to the same buffer, as if they were both sent to "#mychan".

This is necessary due to servers' performing this case folding when reading such messages. Note that IRC only does ASCII case-folding, not Unicode case-folding. Unicode case-folding will not work. See the discussion here: https://todo.sr.ht/~emersion/soju/52.