Avoid forwarding MONITOR requests if upstream doesn't support it

Clients aren't supposed to do this, but in case they do, let's
send them an error.

downstream.go

@@ -2473,6 +2473,9 @@ func (dc *downstreamConn) handleMessageRegistered(ctx context.Context, msg *irc.
 		if uc == nil {
 			return newUnknownCommandError(msg.Command)
 		}
+		if _, ok := uc.isupport["MONITOR"]; !ok {
+			return newUnknownCommandError(msg.Command)
+		}
 
 		var subcommand string
 		if err := parseMessageParams(msg, &subcommand); err != nil {

upstream.go

@@ -2075,6 +2075,10 @@ func (uc *upstreamConn) updateChannelAutoDetach(name string) {
 }
 
 func (uc *upstreamConn) updateMonitor() {
+	if _, ok := uc.isupport["MONITOR"]; !ok {
+		return
+	}
+
 	add := make(map[string]struct{})
 	var addList []string
 	seen := make(map[string]struct{})