fix: ensure buffered events are sent in order

Before this commit, an event sent in the "connect" handler could be
sent before the events that were buffered while disconnected.

```js
socket.on("connect", () => {
  socket.emit("bar");
});

socket.emit("foo"); // buffered while disconnected
```

In the example above, the "bar" event was sent first, which is not
correct.

Related: #1458

Backported from 34f822f

lib/socket.js

@@ -332,8 +332,8 @@ Socket.prototype.onack = function (packet) {
 Socket.prototype.onconnect = function () {
   this.connected = true;
   this.disconnected = false;
-  this.emit('connect');
   this.emitBuffered();
+  this.emit('connect');
 };
 
 /**

test/socket.js

@@ -176,4 +176,24 @@ describe('socket', function () {
       done();
     });
   });
+
+  it('should emit events in order', function (done) {
+    var socket = io('/', { autoConnect: false });
+    var i = 0;
+
+    socket.on('connect', function () {
+      socket.emit('echo', 'second', function () {
+        expect(++i).to.eql(2);
+
+        socket.disconnect();
+        done();
+      });
+    });
+
+    socket.emit('echo', 'first', function () {
+      expect(++i).to.eql(1);
+    });
+
+    socket.connect();
+  });
 });

test/support/server.js

@@ -40,6 +40,10 @@ server.on('connection', function (socket) {
     socket.emit('hi');
   });
 
+  socket.on('echo', function (arg, callback) {
+    callback(arg);
+  });
+
   // ack tests
   socket.on('ack', function () {
     socket.emit('ack', function (a, b) {