Add #[must_use] to Response struct

[JarredAllen]

(feel free to close if you disagree with me that this is a desirable feature)

I just had a bug-hunting session that resulted in me accidentally writing code:

fn handle_request(request: Request) {
    if <"If-None-Match" header with matching ETag was sent> {
        Response::empty(304);
    }
    request.respond(Response::new(..)).unwrap();
}

when I meant to write the code:

fn handle_request(request: Request) {
    if <"If-None-Match" header with matching ETag was sent> {
        request.respond(Response::empty(304)).unwrap();
        return;
    }
    request.respond(Response::new(..)).unwrap();
}

This attribute will prevent that from happening (though it is backwards-incompatible, so this probably should wait for the next major release).