When express app is mounted at non-root endpoint, passport-http returns 400

Mounting an app at a non-root endpoint fails on the uri check because passport-http is only checking the relative uri as seen by the express app, not by the uri as seen by the client.
e.g.

``` javascript
authApp = require('../modules/auth');
app.use('/auth', authApp);

// ../module/auth.js
app.get('/digest', passport.authenticate 'digest');
```

Seems to always yield 400.
