Impossible to access cookies within mutateAndGetPayload
Hello here

At first glance this library looks nice, unless you start working with it and end up stack with poor documentation. I have a very simple and very common use case. I want to handle auth. through session and by having LoginMutation I need to access **ctx** inside it so I can:
1) set cookies
2) access ctx.* objects 

Without such information - it's completely useless.

Right now I've tried to set cookies on 2nd and 3rd argument. Nothing works.
Attempted to pass values through rootValue - same.
It's literally impossible to use that library when everything in Koa is running around ctx

It's either me not understanding something or my case is super rare 

server.js
```js
router.all(
  '/graphql',
  graphqlHTTP((request, ctx) => ({
    schema: graphqlSchema,
    graphiql: true,
    rootValue: { ctx },
    pretty: true,
  })),
)
```

LoginMutation.js
```js
  mutateAndGetPayload: async ({ email, password }, ctx) => {
    ctx.cookies.set('mykEY', '123')
    console.log(`ctx.cookies.set: ${ctx.cookies.get('mykEY')}`) //nothing
}
```

When I do that inside middleware - it works!

server.js
```
app.user(mymw)
```

mymw.js
```js
export default async function sessionMiddleware(ctx, next) {
  console.log(`ctx.cookies.set: ${ctx.cookies.get('mykEY')}`)
  ctx.cookies.set('mykEY', '123') // here
```

//2nd call and it's there