Fix/empty post request (#971)

* add test for empty post request on httpClientWrapper

* convert test method to not have async in signature

* add post method to http wrapper mock call

* add valid url to httpClientWrapperTest

* use AreEquals instead of Equals

* introduce httpClientWrapper fix

Adyen.Test/HttpClientWrapperTest.cs

@@ -0,0 +1,27 @@
+using System.Linq;
+using System.Net.Http;
+using System.Threading.Tasks;
+using Adyen.HttpClient;
+using Adyen.Model.Management;
+using Adyen.Model.TerminalApi;
+using Adyen.Service.Management;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using NSubstitute;
+
+namespace Adyen.Test
+{
+    [TestClass]
+    public class HttpClientWrapperTest : BaseTest
+    {
+        [TestMethod]
+        public void EmptyRequestBodyPostTest()
+        {
+            var mockHttpMessageHandler = new MockHttpMessageHandler("{}", System.Net.HttpStatusCode.OK);
+            var httpClient = new System.Net.Http.HttpClient(mockHttpMessageHandler);
+            var httpClientWrapper = new HttpClientWrapper(MockPaymentData.CreateConfigApiKeyBasedMock(), httpClient);
+            var _ = httpClientWrapper.Request("https://test.com/testpath", null, null, HttpMethod.Post);
+            Assert.AreEqual("{}", mockHttpMessageHandler.Input);
+        }
+
+      }
+}

Adyen.Test/mockHttpClient.cs

@@ -0,0 +1,34 @@
+using System.Net;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+
+public class MockHttpMessageHandler : HttpMessageHandler
+{
+    private readonly string _response;
+    private readonly HttpStatusCode _statusCode;
+
+    public string Input { get; private set; }
+    public int NumberOfCalls { get; private set; }
+
+    public MockHttpMessageHandler(string response, HttpStatusCode statusCode)
+    {
+        _response = response;
+        _statusCode = statusCode;
+    }
+
+    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,
+        CancellationToken cancellationToken)
+    {
+        NumberOfCalls++;
+        if (request.Content != null) // Could be a GET-request without a body
+        {
+            Input = await request.Content.ReadAsStringAsync();
+        }
+        return new HttpResponseMessage
+        {
+            StatusCode = _statusCode,
+            Content = new StringContent(_response)
+        };
+    }
+}

Adyen/HttpClient/HttpClientWrapper.cs

@@ -33,7 +33,7 @@ public async Task<string> RequestAsync(string endpoint, string requestBody, Requ
         {
             if (requestBody == null && (httpMethod == HttpMethod.Post || httpMethod == new HttpMethod("PATCH")))
             {
-                requestBody = "";
+                requestBody = "{}";
             }
 
             using (var request = GetHttpRequestMessage(endpoint, requestBody, requestOptions, httpMethod))