add broadcasting test where 10 clients exchange messages, to try to trigger threading errors

test/IXWebSocketServerTest.cpp

@@ -62,7 +62,7 @@ namespace ix
                     {
                         if (client != webSocket)
                         {
-                            client->send(msg->str);
+                            client->send(msg->str, msg->binary);
                         }
                     }
                 }