Add Content-Type as default header that is set (#186)

This was a default value that it appears some internal teams had relied on and caused problems when they were migrating. This change expands an existing middleware to handle adding any default response headers

Author: twsouthwick

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

@@ -0,0 +1,37 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Net.Http.Headers;
+
+namespace Microsoft.AspNetCore.SystemWebAdapters;
+
+/// <summary>
+/// System.Web would set some response headers by default. This adds those early in the pipeline so they can still be overridden. If they exist, they will not be replaced.
+/// </summary>
+internal class SetDefaultResponseHeadersMiddleware
+{
+    private readonly RequestDelegate _next;
+
+    private readonly (string Header, string Value)[] _defaultHeaders = new (string, string)[]
+    {
+        (HeaderNames.CacheControl, CacheControlHeaderValue.PrivateString),
+        (HeaderNames.ContentType, "text/html")
+    };
+
+    public SetDefaultResponseHeadersMiddleware(RequestDelegate next) => _next = next;
+
+    public Task InvokeAsync(HttpContext context)
+    {
+        foreach (var (header, value) in _defaultHeaders)
+        {
+            if (!context.Response.Headers.ContainsKey(header))
+            {
+                context.Response.Headers[header] = value;
+            }
+        }
+
+        return _next(context);
+    }
+}

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

@@ -29,7 +29,7 @@ public static void UseSystemWebAdapters(this IApplicationBuilder app)
     {
         HttpRuntime.Current = app.ApplicationServices.GetRequiredService<IHttpRuntime>();
 
-        app.UseMiddleware<DefaultCacheControlMiddleware>();
+        app.UseMiddleware<SetDefaultResponseHeadersMiddleware>();
         app.UseMiddleware<PreBufferRequestStreamMiddleware>();
         app.UseMiddleware<SessionMiddleware>();
         app.UseMiddleware<BufferResponseStreamMiddleware>();

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

@@ -0,0 +1,47 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+using Xunit;
+
+namespace Microsoft.AspNetCore.SystemWebAdapters.Tests;
+
+public class SetDefaultResponseHeadersMiddlewareTests
+{
+    [Fact]
+    public async Task NoHeaders()
+    {
+        // Arrange
+        var context = new DefaultHttpContext();
+        var next = Task (HttpContextCore context) => Task.CompletedTask;
+        var middleware = new SetDefaultResponseHeadersMiddleware(new(next));
+
+        // Act
+        await middleware.InvokeAsync(context);
+
+        // Assert
+        Assert.Equal(2, context.Response.Headers.Count);
+        Assert.Equal("text/html", context.Response.Headers.ContentType);
+        Assert.Equal("private", context.Response.Headers.CacheControl);
+    }
+
+    [Theory]
+    [InlineData("Content-Type", "some-content-type")]
+    [InlineData("Cache-Control", "cache-value")]
+    public async Task Existing(string name, string value)
+    {
+        // Arrange
+        var context = new DefaultHttpContext();
+        context.Response.Headers[name] = value;
+
+        var next = Task (HttpContextCore context) => Task.CompletedTask;
+        var middleware = new SetDefaultResponseHeadersMiddleware(new(next));
+
+        // Act
+        await middleware.InvokeAsync(context);
+
+        // Assert
+        Assert.Equal(value, context.Response.Headers[name]);
+    }
+}