code: func (conn *Conn) Join(channel string, key ...string) {
  	k := ""
  	if len(key) > 0 {
  		k = " " + key[0]
  	}
  	conn.Raw(JOIN + " " + channel + k)
  }
docs: // Join sends a JOIN command to the server with an optional key.
  //     JOIN channel [key]