code: public static String preOrder(Map<String, String> params, String privateKey) throws Exception {
  		// 生成签名前先去除sign
  		params.remove("sign");
  		params.put("sign", RSAUtils.encryptByPrivateKey(PaymentKit.packageSign(params, false), privateKey));
  		Map<String, String> headers = new HashMap<String, String>();
  		headers.put("Content-Type", "application/json");
  		return HttpKit.post(PERORDER, JSON.toJSONString(params), headers);
  	}
docs: 统一下单接口
  
  @param params
  @param privateKey
  @return
  @throws Exception