Use the real Request object in subscriptions

[mcollina]

Signed-off-by: Matteo Collina hello@matteocollina.com

This PR does a few things:

1. uses the real Request object in the subscription Context. Reply is still "faked" as there is no Reply for websockets.
2. copies over all the data incoming as part of connection_init packet to req.headers. If there is a headers object that's used instead (as it was before). This change was needed because GraphiQL treats all the payload as headers.

[jonnydgreen]

LGTM - reckon we should add the details/behaviour in the PR description to the docs for future reference?

[mcollina]

Docs added.