Although applications in theory shouldn't care to see connection-level headers (e.g. `Transfer-Encoding`), higher-level specs like the JavaScript Fetch API often specify that these headers should be visible, and they can be useful for debugging. So, this change makes it so that the application can see the connection-level headers on incoming requests.

For outgoing requests, the application can provide an HttpHeaders object that specifies these headers (important especially for the pass-through case), but the HTTP implementation will ignore them.

Additionally, we can now allow the application to set WebSocket connection-level headers on non-WebSocket requests. This is useful for frameworks that emulate WebSocket over HTTP and assume the ability to set WebSocket headers (especially `Sec-WebSocket-Extension`) on regular non-WebSocket HTTP requests.