Add a unit test and an integration test for the fix.

test/WireMock.Net.Tests/Http/HttpRequestMessageHelperTests.cs

@@ -49,6 +49,26 @@ public async Task HttpRequestMessageHelper_Create_Bytes()
         Check.That(await message.Content.ReadAsByteArrayAsync().ConfigureAwait(false)).ContainsExactly(Encoding.UTF8.GetBytes("hi"));
     }
 
+    [Fact]
+    public async Task HttpRequestMessageHelper_Create_TextPlain()
+    {
+        // Assign
+        var body = new BodyData
+        {
+            BodyAsString = "0123", // or 83 in decimal
+            BodyAsJson = 83,
+            DetectedBodyType = BodyType.Json,
+            DetectedBodyTypeFromContentType = BodyType.String
+        };
+        var request = new RequestMessage(new UrlDetails("http://localhost/foo"), "GET", ClientIp, body);
+
+        // Act
+        var message = HttpRequestMessageHelper.Create(request, "http://url");
+
+        // Assert
+        Check.That(await message.Content!.ReadAsStringAsync().ConfigureAwait(false)).Equals("0123");
+    }
+
     [Fact]
     public async Task HttpRequestMessageHelper_Create_Json()
     {
@@ -64,7 +84,7 @@ public async Task HttpRequestMessageHelper_Create_Json()
         var message = HttpRequestMessageHelper.Create(request, "http://url");
 
         // Assert
-        Check.That(await message.Content.ReadAsStringAsync().ConfigureAwait(false)).Equals("{\"x\":42}");
+        Check.That(await message.Content!.ReadAsStringAsync().ConfigureAwait(false)).Equals("{\"x\":42}");
     }
 
     [Fact]

test/WireMock.Net.Tests/ResponseBuilders/ResponseWithProxyIntegrationTests.cs

@@ -0,0 +1,106 @@
+#if NET8_0_OR_GREATER
+
+using System;
+using System.IO;
+using System.Linq;
+using System.Net;
+using System.Net.Http;
+using System.Net.Http.Headers;
+using System.Text;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Hosting.Server;
+using Microsoft.AspNetCore.Hosting.Server.Features;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using WireMock.Net.Xunit;
+using WireMock.RequestBuilders;
+using WireMock.ResponseBuilders;
+using WireMock.Server;
+using WireMock.Settings;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace WireMock.Net.Tests.ResponseBuilders;
+
+public sealed class ResponseWithProxyIntegrationTests(ITestOutputHelper output)
+{
+    [Fact]
+    public async Task Response_UsingTextPlain()
+    {
+        // Given
+        using var server = await TestServer.New().Run();
+        var port = server.GetPort();
+        output.WriteLine($"Server running on port {port}");
+
+        var settings = new WireMockServerSettings {
+            Port = 0,
+            Logger = new TestOutputHelperWireMockLogger(output)
+        };
+        using var mockServer = WireMockServer.Start(settings);
+        mockServer.Given(Request.Create().WithPath("/zipcode").UsingPatch())
+                  .RespondWith(Response.Create().WithProxy($"http://localhost:{port}"));
+
+        using var client = new HttpClient { BaseAddress = new Uri(mockServer.Urls[0]) };
+        using var content = new ByteArrayContent(Encoding.UTF8.GetBytes("0123"));
+        content.Headers.ContentType = new MediaTypeHeaderValue("text/plain");
+
+        // When
+        var response = await client.PatchAsync("/zipcode", content);
+
+        // Then
+        response.StatusCode.Should().Be(HttpStatusCode.OK);
+        response.Content.Headers.GetValues("Content-Type").Should().BeEquivalentTo("text/plain; charset=utf-8");
+        var result = await response.Content.ReadAsStringAsync();
+        result.Should().Be("0123");
+    }
+
+    sealed class Disposable(Action dispose) : IDisposable
+    {
+        public void Dispose() => dispose();
+    }
+
+    sealed class TestServer(WebApplication app) : IDisposable
+    {
+        Disposable disposable = new(() => { });
+
+        public static TestServer New() {
+            var builder = WebApplication.CreateBuilder();
+            builder.WebHost.ConfigureKestrel(opts => opts.ListenAnyIP(0));
+
+            var app = builder.Build();
+
+            app.MapPatch("/zipcode", async (HttpRequest req) => {
+                var memory = new MemoryStream();
+                await req.Body.CopyToAsync(memory);
+                var content = Encoding.UTF8.GetString(memory.ToArray());
+                return content;
+            });
+            return new(app);
+        }
+
+        public int GetPort()
+            => app.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>()!.Addresses
+                  .Select(x => new Uri(x).Port)
+                  .First();
+
+        public async ValueTask<TestServer> Run() {
+            var started = new TaskCompletionSource();
+            var host = app.Services.GetRequiredService<IHostApplicationLifetime>();
+            host.ApplicationStarted.Register(() => started.SetResult());
+            _ = Task.Run(() => app.RunAsync());
+            await started.Task;
+            disposable = new(() => host.StopApplication());
+            return this;
+        }
+
+        public void Dispose() {
+            disposable.Dispose();
+        }
+    }
+}
+
+#endif