Don't force default content-type if no content

src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/SetDefaultResponseHeadersMiddleware.cs

@@ -32,7 +32,7 @@ public Task InvokeAsync(HttpContext context)
 
     private static void WriteDefaultContentType(HttpContext context)
     {
-        if (context.Response.Headers.ContentType.Count == 0)
+        if (context.Response.ContentLength.HasValue && context.Response.Headers.ContentType.Count == 0)
         {
             context.Response.Headers.ContentType = "text/html";
         }

test/Microsoft.AspNetCore.SystemWebAdapters.CoreServices.Tests/SetDefaultResponseHeadersMiddlewareTests.cs

@@ -2,6 +2,8 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.IO;
+using System.Text;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
@@ -12,23 +14,46 @@ namespace Microsoft.AspNetCore.SystemWebAdapters.Tests;
 public class SetDefaultResponseHeadersMiddlewareTests
 {
     [Fact]
-    public async Task NoHeaders()
+    public async Task NoHeadersWithContent()
     {
         // Arrange
         var context = new DefaultHttpContext();
         var startupFeature = new StartupCallbackFeature();
         context.Features.Set<IHttpResponseFeature>(startupFeature);
         var next = Task (HttpContextCore context) => Task.CompletedTask;
         var middleware = new SetDefaultResponseHeadersMiddleware(new(next));
+        var data = new MemoryStream(Encoding.UTF8.GetBytes("ArbitraryContent"));
+        context.Response.Body = data;
+        context.Response.ContentLength = data.Length;
 
         // Act
         await middleware.InvokeAsync(context);
         await startupFeature.RunAsync();
 
         // Assert
-        Assert.Equal(2, context.Response.Headers.Count);
+        Assert.Equal(3, context.Response.Headers.Count);
         Assert.Equal("text/html", context.Response.Headers.ContentType);
         Assert.Equal("private", context.Response.Headers.CacheControl);
+        Assert.Equal(data.Length, context.Response.ContentLength);
+    }
+
+    [Fact]
+    public async Task NoHeadersWithoutContent()
+    {
+        // Arrange
+        var context = new DefaultHttpContext();
+        var startupFeature = new StartupCallbackFeature();
+        context.Features.Set<IHttpResponseFeature>(startupFeature);
+        var next = Task (HttpContextCore context) => Task.CompletedTask;
+        var middleware = new SetDefaultResponseHeadersMiddleware(new(next));
+
+        // Act
+        await middleware.InvokeAsync(context);
+        await startupFeature.RunAsync();
+
+        // Assert
+        Assert.Single(context.Response.Headers);
+        Assert.False(context.Response.Headers.ContainsKey("Content-Type"));
     }
 
     [Theory]