Hapi v17+ path prefix not working
Hello, I'm trying to make a twitter integration work, look at this code:

```js
var Hapi = require('hapi')
var yar = require('yar')
var grant = require('grant-hapi')

const PATH_PREFIX = '/prefix'

var server = new Hapi.Server({ host: 'localhost', port: 3000 })

server.realm.modifiers.route.prefix = PATH_PREFIX

server.route({
  method: 'GET', path: ('/hi'), handler: (req, res) => {
    return res.response("session: " + JSON.stringify(req.auth) + "\n" + JSON.stringify(req.query, null, 2)).header('content-type', 'text/plain')
  }
})

server.register([
  { plugin: yar, options: { cookieOptions: { password: '01234567890123456789012345678912', isSecure: false } } },
  {
    routes: { prefix: PATH_PREFIX },
    plugin: grant(), options: {
      "defaults": {
        "protocol": "http",
        "host": "localhost:3000",
        "prefix": PATH_PREFIX
      },
      "twitter": {
        "key": "xxxxxxx",
        "secret": "yyyyy",
        "callback": PATH_PREFIX + "/hi"
      }
    }
  }
])
  .then(() => server.start())
```

When I navigate to:
`http://localhost/prefix/connect/twitter` 

the flow works but ends up into: 
`http://localhost:3000/connect/twitter/callback?oauth_token=XXXXXXX`

 instead of:
`http://localhost:3000/prefix/connect/twitter/callback?oauth_token=XXXXXXX`.

Why is that?