Grant does not work with Koa 2
Grant is incompatible with Koa 2 (which is the current version of Koa).

This is the code I'm using to test:

```js
const Koa = require('koa');
const session = require('koa-session');
const mount = require('koa-mount');

const Grant = require('grant-koa');

const app = new Koa();
app.listen(8000);

app.keys = [ 'b39bfe95' ];

app.use(session(app));

const grant = new Grant({
  server: {
    protocol: 'http',
    host: 'localhost:8000',
  },
  github: {
    key: '[CLIENT_ID]',
    secret: '[CLIENT_SECRET]',
    scope: [ 'user:email', 'repo' ],
  },
});
app.use(mount(grant));
```

When using old versions of the dependencies (`koa@1`, `koa-mount@1`, `koa-session@1`), this code works as expected when accessing `/connect/github`, but when using the newest versions of each dependency, I only get the message `Not Found` (as if Koa ignores the Grant completely).

I've tried replacing `github` with other providers (such as `facebook`), to no avail.

Here's the complete list of versions:

```json
{
  "dependencies": {
    "grant-koa": "3.7.1",
    "koa": "2.2.0",
    "koa-mount": "3.0.0",
    "koa-session": "5.0.0"
  }
}
```