passport facebook route 404 error
Versions

```
"koa-passport": "^1.3.0"
"koa-router": "^5.4.0"
NodeJS 6.1.0
```

I get 404 error when use koa-passport and koa-router in my app
my route file

``` javascript
const Router = require('koa-router');
const passport = require('koa-passport');

function register (app) {
    const router = new Router({
        prefix: '/auth'
    });
    router.get('/facebook', function* () {
        passport.authenticate('facebook');
    });
    app.use(router.routes());
    app.use(router.allowedMethods());
}

module.exports = register;
```

app.js

``` javascript
app.use(passport.initialize());
app.use(passport.session());
//....
require('./app/server/routes/auth.route')(app);
```

When I go to the '/auth/facebook' route I get 'Page not Found' error.
If I comment passport.authenticate('facebook'); line route works fine with something like this.body = 'facebook login';

Thanks
