Verifies all invariant- and required headers

        If a header is missing or have an incorrect value ValueError will be
        raised

def _handle_websocket_headers(self, handler: WebSocketHandler) -> None:
        """Verifies all invariant- and required headers

        If a header is missing or have an incorrect value ValueError will be
        raised
        """
        fields = ("Host", "Sec-Websocket-Key", "Sec-Websocket-Version")
        if not all(map(lambda f: handler.request.headers.get(f), fields)):
            raise ValueError("Missing/Invalid WebSocket headers")