Send a custom HTTP header for "101 Upgrade" response

[micolous]

I need to be able to send a custom HTTP header in the 101 Upgrade response (as part of WebSocket connection initialisation), around about here:

tide-websockets/src/handler.rs

Lines 175 to 180 in c3e1d98

	if let Some(protocol) = protocol {
	response.insert_header("Sec-Websocket-Protocol", protocol);
	}
	let http_res: &mut tide::http::Response = response.as_mut();
	let upgrade_receiver = http_res.recv_upgrade().await;

It'd be nice to be able to have some extension to the handler so that my code can do some work to add this header in, and have some extra connection-level state along with the WebSocketStream.