Don't directly reply to network-specific NICK

The NICK must only apply to the specific network, not to the downstream
connection.

git-svn-id: file:///srv/svn/repo/suika/trunk@512 f0ae65fe-ee39-954e-97ec-027ff2717ef4

downstream.go

@@ -1201,7 +1201,7 @@ func (dc *downstreamConn) handleMessageRegistered(msg *irc.Message) error {
 			})
 		})
 
-		if dc.upstream() == nil && dc.nick != nick {
+		if dc.upstream() == nil && upstream == nil && dc.nick != nick {
 			dc.SendMessage(&irc.Message{
 				Prefix:  dc.prefix(),
 				Command: "NICK",