签名添加/修改接口，支持同一请求传多次license_url或license_base64参数，以实现上传多个证明文件的需求。

以下为添加签名接口license_url传多次的代码示例：

public static void main(String[] args) throws Exception {
    String url = "http://sms.yunpian.com/v2/sign/add.json";
    try (CloseableHttpClient client = HttpClients.createDefault()) {
      HttpPost httpPost = new HttpPost(url);
      httpPost.setHeader("Content-Type", "application/x-www-form-urlencoded;charset=utf-8");
      List<NameValuePair> params = new ArrayList<>();
      params.add(new BasicNameValuePair("sign", "云片测试"));
      params.add(new BasicNameValuePair("website", "https://www.yunpian.com"));
      params.add(new BasicNameValuePair("license_url", "http://xxx1.jpg"));
      params.add(new BasicNameValuePair("license_url", "http://xxx2.jpg"));
      params.add(new BasicNameValuePair("apikey", "xxx"));

      httpPost.setEntity(new UrlEncodedFormEntity(params, "UTF-8"));
      try (CloseableHttpResponse response = client.execute(httpPost)) {
        String responseBody = EntityUtils.toString(response.getEntity(), "UTF-8");
        System.out.println(responseBody);
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
  }