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

lib/manager.ts

@@ -520,13 +520,7 @@ export class Manager<
     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();
   }
 
@@ -545,7 +539,7 @@ export class Manager<
    * @private
    */
   private onclose(reason: string): void {
-    debug("onclose");
+    debug("closed due to %s", reason);
 
     this.cleanup();
     this.backoff.reset();

test/socket.ts

@@ -231,6 +231,25 @@ describe("socket", function () {
     }, 100);
   });
 
+  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);
+      success(done, socket);
+    }, 200);
+  });
+
   it("should throw on reserved event", () => {
     const socket = io("/no", { forceNew: true });