Read response object after koa-graphql has run
I'd like to read the response object that's going to be sent back to the requestor. When I interrogate the `response` object that's passed when providing a function to `koa-graphql`,  it seems it's before the final response has been composed.

i.e.
```
function middleware() {
 return graphqlHTTP(
    (request: Object, response: Object, params: Object): Object => {
      // read the response that will be provided to the requestor
      // the response at this point in time is incomplete, it seems. In fact, the status code will be 404.
      return {
        schema,
        graphiql: enableGraphiql || isNotProd,
        formatError: errorFormatterWithMetadata,
        pretty: prettifyJsonResponse,
        extensions: extensionWithMetadata,
        validationRules: additionalValidation,
      }
    },
  )
 }
}