docs: protected final void selectSubprotocol(final WebSocketHttpExchange exchange) {
          String requestedSubprotocols = exchange.getRequestHeader(Headers.SEC_WEB_SOCKET_PROTOCOL_STRING);
          if (requestedSubprotocols == null) {
              return;
          }
  
          String[] requestedSubprotocolArray = PATTERN.split(requestedSubprotocols);
          String subProtocol = supportedSubprotols(requestedSubprotocolArray);
          if (subProtocol != null && !subProtocol.isEmpty()) {
              exchange.setResponseHeader(Headers.SEC_WEB_SOCKET_PROTOCOL_STRING, subProtocol);
          }
  
      }