Added request.agent(app) support

I added `request.agent(app)` support.
See following test:

```js
describe('request.agent(app)', function(){
  var app = express();

  app.use(express.cookieParser());

  app.get('/', function(req, res){
    res.cookie('cookie', 'hey');
    res.send();
  });

  app.get('/return', function(req, res){
    if(req.cookies.cookie) res.send(req.cookies.cookie);
    else res.send(':(')
  });

  var agent = request.agent(app);

  it('should save cookies', function(done){
    agent
    .get('/')
    .expect('set-cookie', 'cookie=hey; Path=/', done);
  })

  it('should send cookies', function(done){
    agent
    .get('/return')
    .expect('hey', done);
  })
})
```