bugfix: has two connect on connection callback

[mtdxc]

i have woked SioChatDemo with socket.io chatroom example
and found if i connect with default namespace just for one client, node server will got two connection callback.
it sounds like default namespace don't need to call send connection packet, server will handle it.
and then i make a fixed.

here i node.js server code, just add a console.log at connection callback..

io.on('connection', (socket) => {
  let addedUser = false;
  **console.log("new connection");**
  // when the client emits 'new message', this listens and executes
  socket.on('new message', (data) => {
    // we tell the client to execute 'new message'
    socket.broadcast.emit('new message', {
      username: socket.username,
      message: data
    });
  });
  ...