fix a DoS against websocket clients

I assumed gorilla validated UTF8 for incoming text messages. In fact, the
documentation states:

>It is the application's responsibility to ensure that text messages
>are valid UTF-8 encoded text.

and this applies to both incoming and outgoing messages. Consequently,
even when enforce-utf8 is enabled, it was possible to send invalid UTF8
to Ergo inside a websocket text frame. This data would be incorrectly
considered valid UTF8, and could be relayed to other clients, including
to websocket clients inside a text frame. The resulting frame would violate
the websocket protocol, causing web clients to be disconnected.

irc/ircconn.go

 }
 
 func (wc IRCWSConn) ReadLine() (line []byte, err error) {
-	messageType, line, err := wc.conn.ReadMessage()
+	_, line, err = wc.conn.ReadMessage()
 	if err == nil {
-		if messageType == websocket.BinaryMessage && !utf8.Valid(line) {
+		if !utf8.Valid(line) {
 			return line, errInvalidUtf8
 		}
 		return line, nil