NanoWSD.java

 protected boolean isWebsocketRequested(IHTTPSession session) {
        Map<String, String> headers = session.getHeaders();
        String upgrade = headers.get(NanoWSD.HEADER_UPGRADE);
        boolean isCorrectConnection = isWebSocketConnectionHeader(headers);
        boolean isUpgrade = NanoWSD.HEADER_UPGRADE_VALUE.equalsIgnoreCase(upgrade);
        return isUpgrade && isCorrectConnection;
    }