fix: merge Engine.IO options

So that the following example:

```js
const io = require('socket.io')({
  pingTimeout: 10000
});

io.listen(3000);
```

behaves the same as:

```js
const io = require('socket.io')(3000, {
  pingTimeout: 10000
});
```

Before this change, the options in the first example were not forwarded
to the Engine.IO constructor, which is not really intuitive.

The previous syntax (which is still valid):

```js
const io = require('socket.io')();

io.listen(3000, {
  pingTimeout: 10000
});
```

lib/index.ts

@@ -169,6 +169,7 @@ export class Server extends EventEmitter {
   > = new Map();
   private _adapter: any;
   private _serveClient: boolean;
+  private opts: Partial<EngineOptions>;
   private eio;
   private engine;
   private _path: string;
@@ -203,7 +204,8 @@ export class Server extends EventEmitter {
     this.encoder = new this._parser.Encoder();
     this.adapter(opts.adapter || Adapter);
     this.sockets = this.of("/");
-    if (srv) this.attach(srv, opts);
+    this.opts = opts;
+    if (srv) this.attach(srv);
   }
 
   /**
@@ -357,6 +359,8 @@ export class Server extends EventEmitter {
       srv.listen(port);
     }
 
+    // merge the options passed to the Socket.IO server
+    Object.assign(opts, this.opts);
     // set engine.io path to `/socket.io`
     opts.path = opts.path || this._path;
 

test/socket.io.ts

@@ -138,6 +138,24 @@ describe("socket.io", () => {
             done();
           });
       });
+
+      it("should work with #attach (and merge options)", () => {
+        const srv = createServer((req, res) => {
+          res.writeHead(404);
+          res.end();
+        });
+        const server = new Server({
+          pingTimeout: 6000
+        });
+        server.attach(srv, {
+          pingInterval: 24000
+        });
+        // @ts-ignore
+        expect(server.eio.opts.pingTimeout).to.eql(6000);
+        // @ts-ignore
+        expect(server.eio.opts.pingInterval).to.eql(24000);
+        server.close();
+      });
     });
 
     describe("port", () => {