Transport: querystring redirecting to /
Hi!

I tried the [express transport-querystring example](https://github.com/simov/grant/blob/master/examples/transport-querystring/express.js) with the following configuration:
```
{
  "defaults": {
    "origin": "http://localhost:3000",
    "transport": "querystring"
  },
  "zoom": {
    "key": "...",
    "secret": "...",
  }
}
```
and the correct express handler
```
var express = require('express')
var session = require('express-session')
var grant = require('../../').express()

express()
  .use(session({secret: 'grant', saveUninitialized: true, resave: false}))
  .use(grant(require('./config.json')))
  .get('/connect/zoom/callback', (req, res) => {
    res.end(JSON.stringify(req.query, null, 2))
  })
  .listen(3000)
```

And the callback route `http://localhost:3000/connect/zoom/callback` redirects to `http://localhost:3000/`.

Everything works well if I change to `session` transport, and I think it might be related to how the session is handled, maybe the express-session package?, but I can't find it.

Is there a bug here or am I configuring something wrong?

Thanks!