def setacl(mailbox, user, rights)
      if rights.nil?
        send_command("SETACL", mailbox, user, "")
      else
        send_command("SETACL", mailbox, user, rights)
      end
    end
Sends the SETACL command along with +mailbox+, +user+ and the
+rights+ that user is to have on that mailbox.  If +rights+ is nil,
then that user will be stripped of any rights to that mailbox.
The IMAP ACL commands are described in [RFC-2086].