HTTP digest on router

Hello,

I am unable to apply a `DigestStrategy` to a route defined in a express' `Router`.

[Here's the following snippet](https://gist.github.com/Gawen/ed8663a6d2d543c3ab29391d12bdb55f#file-server-js). Note the variable `use_router`.

```js
var handler = function(req, res) {
res.json({"hello": "world"});
};

if (use_router) {
var router = express.Router();
router.get('/', passport.authenticate('digest', {session: false}), handler);
app.use("/hello", router);
} else {
app.get('/hello', passport.authenticate('digest', {session: false}), handler);
}
```

I expected to received `{"hello": "world"}` if I `GET` on `/hello` the right username (any value) and password (`"password"`).

```bash
curl -v --user user:password --digest http://localhost:8888/hello
```

[It does when `use_router` is `false`](https://gist.github.com/Gawen/ed8663a6d2d543c3ab29391d12bdb55f#file-use_router_is_false_output-txt), BUT [it does not when `use_router` is `true`](https://gist.github.com/Gawen/ed8663a6d2d543c3ab29391d12bdb55f#file-use_router_is_true_output-txt).

I would expect aaplying a `DigestStrategy` to a route registered in a router to work, but it appears it does not.

Used versions:

- express: "^4.14.0"
- passport: "^0.3.2"
- passport-http: "^0.3.0"

Start with the linked server.js example and compare the DigestStrategy behavior for the direct route and the Express Router route. Reproduce both curl requests and trace the router-mounted authentication entry point; done means the router-based /hello request returns the expected JSON with digest authentication.

Written by the indexing model from the issue text.