koa access_token missing from session
I cloned the `examples/koa-session-stores` example, filled in my provider's details and received the following error, despite the flow working successfully, indicating the the `grant` attribute was never added to the `session`:
```
Koa server listening on port 3000

  TypeError: Cannot read property 'response' of undefined
      at Object.<anonymous> (/private/var/tmp/grant/examples/koa-session-stores/koa-session.js:23:51)
      at Generator.next (<anonymous>)
      at Object.dispatch (/private/var/tmp/grant/examples/koa-session-stores/node_modules/koa-router/lib/router.js:317:7)
      at dispatch.next (<anonymous>)
      at Object.allowedMethods (/private/var/tmp/grant/examples/koa-session-stores/node_modules/koa-router/lib/router.js:356:5)
      at allowedMethods.next (<anonymous>)
      at onFulfilled (/private/var/tmp/grant/examples/koa-session-stores/node_modules/co/index.js:65:19)
      at /private/var/tmp/grant/examples/koa-session-stores/node_modules/co/index.js:54:5
      at new Promise (<anonymous>)
      at Object.co (/private/var/tmp/grant/examples/koa-session-stores/node_modules/co/index.js:50:10)
```

If I change the transport to `querystring`, the `access_token` is available in `this.query`. The exact same `config.json` works with the `examples/session-transport` example (which uses express) but I can't seem to get the `access_token` to be properly stored in the session object with the koa-session example. It looks looks like the session object contains only the state, nonce, provider, and expiration. Very possible I'm misunderstanding the docs somewhere along the way.