fix: fix socket.disconnect().connect() usage

Previously, calling `socket.disconnect().connect()` could, if the
connection was upgraded to WebSocket, result in "disconnect" being
emitted twice, and an engine being leaked.

Here's what happened:

> socket.disconnect()

- calls `socket.destroy()` so the socket doesn't listen to the manager events anymore
- then calls `manager._close()` which closes the underlying engine but not the manager itself (it waits for the "close" event of the engine)

> socket.connect()

- calls `socket.subEvents()` so the socket does listen to the manager events
- calls `manager.open()` which creates a new engine

And then the first engine emits a "close" event, which is forwarded to
the socket, hence the second "disconnect" event.

Related: #1014

Backported from 99c2cb8

lib/manager.js

@@ -481,13 +481,7 @@ Manager.prototype.disconnect = function () {
   debug('disconnect');
   this.skipReconnect = true;
   this.reconnecting = false;
-  if ('opening' === this.readyState) {
-    // `onclose` will not fire because
-    // an open event never happened
-    this.cleanup();
-  }
-  this.backoff.reset();
-  this.readyState = 'closed';
+  this.onclose('forced close');
   if (this.engine) this.engine.close();
 };
 
@@ -498,7 +492,7 @@ Manager.prototype.disconnect = function () {
  */
 
 Manager.prototype.onclose = function (reason) {
-  debug('onclose');
+  debug('closed due to %s', reason);
 
   this.cleanup();
   this.backoff.reset();

test/socket.js

@@ -176,4 +176,24 @@ describe('socket', function () {
       done();
     });
   });
+
+  it('should properly disconnect then reconnect', (done) => {
+    const socket = io('/', { forceNew: true, transports: ['websocket'] });
+
+    let count = 0;
+
+    socket.once('connect', () => {
+      socket.disconnect().connect();
+    });
+
+    socket.on('disconnect', () => {
+      count++;
+    });
+
+    setTimeout(() => {
+      expect(count).to.eql(1);
+      socket.disconnect();
+      done();
+    }, 200);
+  });
 });