graphql-koa-example: subscriptions not working

[paulwib]

Koa will immediately close the connection if ctx.body is not a stream, which it won't be as helix is using the native Node.js response.

This could be avoided by explicitly stopping Koa from handling the response - they warn it's a "hack", but given it's unlikely any Koa middleware is going to further process a GraphQL response it might be OK for the purposes of an example.