[Modify] Polish it

websocket-sharp/Server/WebSocketBehavior.cs

@@ -444,12 +444,15 @@ namespace WebSocketSharp.Server
     private void onOpen (object sender, EventArgs e)
     {
       _id = _sessions.Add (this);
+
       if (_id == null) {
         _websocket.Close (CloseStatusCode.Away);
+
         return;
       }
 
       _startTime = DateTime.Now;
+
       OnOpen ();
     }