: append(mailbox, message, flags = nil, date_time = nil)
      Sends a APPEND command to append the message to the end of
      the mailbox.

      ex).
        imap.append("inbox", <<EOF.gsub(/\n/, "\r\n"), [:Seen], Time.now)
        Subject: hello
        From: shugo@ruby-lang.org
        To: shugo@ruby-lang.org
        
        hello world
        EOF

