我正在尝试使用 Http 包对我的 Flutter 应用程序执行 POST 请求。我首先在 Api 沙盒网站上测试了我的请求，然后在 Postman 中测试了我的请求。它在那里工作得很好，但是一旦在 Flutter 中，我总是收到 400 Bad Request。
这是我在 Flutter 中的代码：
import 'package:http/http.dart';
import 'package:uuid/uuid.dart';
import 'package:wave_app/env/secrets.dart';
import 'package:wave_app/models/momo_token.dart';
String url = "https://sandbox.momodeveloper.mtn.com/collection/v1_0/requesttopay";
var uuid = Uuid();
String requestId = uuid.v4();
MomoToken token = await _createMomoNewTokenCollection();
String auth = "Bearer " + token.accessToken;
Map<String, String> headers = {
"Authorization": auth,
"X-Target-Environment": "sandbox",
"X-Reference-Id": requestId,
"Content-Type": "application/json",
"Ocp-Apim-Subscription-Key": momoCollectionSubscriptionKey
};
String jsonBody = '{"amount": "5","currency": "EUR", "externalId": "123", "payer": {"partyIdType": "MSISDN","partyId": "46733123454"}, "payerMessage": …