POST https://api.weixin.qq.com/wxa/getwxacodeunlimit?access_token=ACCESS_TOKEN 
调用这个接口返回的转化为图片打不开

String token = getAccessToken();
        String url = String.format(wechatConfig.getUnlimitedQRCodeURL(), token);
        String str = HttpClientUtil.post(url, json);
        String filename ="C:/Users/86156/Downloads/" +"微信小程序二维码" + System.currentTimeMillis() + ".png";
//        InputStream inputStream = new ByteArrayInputStream(str.getBytes());
        File file = new File(filename);
        PrintWriter writer = new PrintWriter(file);
        writer.println(str);
        writer.close();
        OssResMessage ossResMessage = ossSdkUtil.uploadFile(inputStream, filename);
        log.info("微信二维码上传oss:{}", ossResMessage);
        return ossResMessage.getPublicCdnHttp();