Handler middleware(Handler innerHandler) {

    return (Request req) async {
        if(req.method == "GET" && req.headers["xxx"] == yyy) {
            // Handle the request
            ...
        }
        else {
            // This gives exception:
            // Bad state: The 'read' method can only be called once on a shelf.Request/shelf.Response object.
            return innerHandler(req);
        }
    }