Support HTTP upgrades (WebSocket etc.)

[clue]

This library should provide support for HTTP upgrades (WebSocket etc.) using the Connection: upgrade and Upgrade: websocket headers.

Among others, this currently depends on reactphp/http-client#145 and a dedicated API to expose the resulting HTTP stream as a DuplexStreamInterface instead of just a ReadableStreamInterface.

[clue]

I have to close this one as per #177 now that future development will focus on https://github.com/reactphp/http instead.

I've migrated this ticket to reactphp/http#376.