WithCallback should use also use enum HttpStatusCode (#535)

* Fix #533

* simplyfy code

test/WireMock.Net.Tests/WireMockServerTests.WithCallback.cs

@@ -0,0 +1,37 @@
+using System.Net;
+using System.Net.Http;
+using System.Threading.Tasks;
+using FluentAssertions;
+using WireMock.RequestBuilders;
+using WireMock.ResponseBuilders;
+using WireMock.Server;
+using Xunit;
+
+namespace WireMock.Net.Tests
+{
+	public partial class WireMockServerTests
+	{
+		[Theory]
+		[InlineData(HttpStatusCode.Conflict)]
+		[InlineData(409)]
+		[InlineData("409")]
+		public async Task WireMockServer_WithCallback_Should_Use_StatusCodeFromResponse(object statusCode)
+		{
+			// Arrange
+			var server = WireMockServer.Start();
+			server.Given(Request.Create().UsingPost().WithPath("/foo"))
+				.RespondWith(Response.Create()
+					.WithCallback(request => new ResponseMessage
+					{
+						StatusCode = statusCode
+					}));
+
+			// Act
+			using var httpClient = new HttpClient();
+			var response = await httpClient.PostAsync("http://localhost:" + server.Ports[0] + "/foo", new StringContent("dummy"));
+
+			// Assert
+			response.StatusCode.Should().Be(409);
+		}
+	}
+}