ADFS integration
Greetings,

I've been trying to make grant work with adfs.

this is my configuration (please do not try to add secret - it won't work):
```
"adfs": {
        "authorize_url": "https://example.com/adfs/oauth2/authorize",
        "access_url": "https://example.com/adfs/oauth2/token",
        "oauth": 2,
        "key": "abc123",
        "redirect_uri": "http://localhost:5000/callback"
      }
```

this is the callback proxy:
```
app.use('/callback', (req, res) => {
  console.log(req);
  if (req.session.grant &&
      req.query.code
  ) {
    res.redirect(`/oauth/${req.session.grant.provider}/callback?${qs.stringify(req.query)}`)
  }
});
```

The error given when using proxy is `error: Cannot read property 'grant' of undefined` that means that req.session is not defined.

When i try to manually take get params and call /adfs/callback with them i get `error=Grant%3A%20missing%20session%20or%20misconfigured%20provider`

Params returned in query string by ADFS are `code` and `client-request-id`

I guess the issue is the missing session. Any ideas how to fix/workaround?

BTW I'm using https://www.npmjs.com/package/@feathersjs/authentication-oauth
But this issue seems to be directly related to grant - correct me if I'm wrong

Thanks for your time
