Fix not properly marshaling self in single-server mode

In single-server mode, we don't need to add a /network suffix when
marshaling, but we still need to replace our nick with the downstream
nick.

downstream.go

@@ -155,15 +155,15 @@ func isOurNick(net *network, nick string) bool {
 // This involves adding a "/<network>" suffix if the entity isn't the current
 // user.
 func (dc *downstreamConn) marshalEntity(net *network, name string) string {
+	if isOurNick(net, name) {
+		return dc.nick
+	}
 	if dc.network != nil {
 		if dc.network != net {
 			panic("soju: tried to marshal an entity for another network")
 		}
 		return name
 	}
-	if isOurNick(net, name) {
-		return dc.nick
-	}
 	return name + "/" + net.GetName()
 }