Fix managed implementation handling of ContentType

Move the Windows implementation to be shared and remove the managed-specific implementation.

Author: stephentoub

src/System.Net.HttpListener/src/System/Net/HttpListenerResponse.cs

@@ -27,6 +27,23 @@ public WebHeaderCollection Headers
 
         public Encoding ContentEncoding { get; set; }
 
+        public string ContentType
+        {
+            get => Headers[HttpKnownHeaderNames.ContentType];
+            set
+            {
+                CheckDisposed();
+                if (string.IsNullOrEmpty(value))
+                {
+                    Headers.Remove(HttpKnownHeaderNames.ContentType);
+                }
+                else
+                {
+                    Headers.Set(HttpKnownHeaderNames.ContentType, value);
+                }
+            }
+        }
+
         public CookieCollection Cookies
         {
             get => _cookies ?? (_cookies = new CookieCollection());

src/System.Net.HttpListener/src/System/Net/Managed/HttpListenerResponse.Managed.cs

@@ -39,7 +39,6 @@ public sealed partial class HttpListenerResponse : IDisposable
     {
         private long _contentLength;
         private bool _clSet;
-        private string _contentType;
         private bool _keepAlive = true;
         private HttpResponseStream _outputStream;
         private Version _version = HttpVersion.Version11;
@@ -74,18 +73,6 @@ public long ContentLength64
             }
         }
 
-        public string ContentType
-        {
-            get => _contentType;
-            set
-            {
-                CheckDisposed();
-                CheckSentHeaders();
-
-                _contentType = value;
-            }
-        }
-
         public bool KeepAlive
         {
             get => _keepAlive;
@@ -247,11 +234,6 @@ internal void SendHeaders(bool closing, MemoryStream ms, bool isWebSocketHandsha
         {
             if (!isWebSocketHandshake)
             {
-                if (_contentType != null)
-                {
-                    _webHeaders.Set(HttpKnownHeaderNames.ContentType, _contentType);
-                }
-
                 if (_webHeaders[HttpKnownHeaderNames.Server] == null)
                     _webHeaders.Set(HttpKnownHeaderNames.Server, HttpHeaderStrings.NetCoreServerName);
                 CultureInfo inv = CultureInfo.InvariantCulture;

src/System.Net.HttpListener/src/System/Net/Windows/HttpListenerResponse.Windows.cs

@@ -56,23 +56,6 @@ internal HttpListenerResponse(HttpListenerContext httpContext) : this()
 
         private HttpListenerRequest HttpListenerRequest => HttpListenerContext.Request;
 
-        public string ContentType
-        {
-            get => Headers[HttpKnownHeaderNames.ContentType];
-            set
-            {
-                CheckDisposed();
-                if (string.IsNullOrEmpty(value))
-                {
-                    Headers.Remove(HttpKnownHeaderNames.ContentType);
-                }
-                else
-                {
-                    Headers.Set(HttpKnownHeaderNames.ContentType, value);
-                }
-            }
-        }
-
         public Stream OutputStream
         {
             get

src/System.Net.HttpListener/tests/HttpListenerResponseTests.Headers.cs

@@ -110,8 +110,7 @@ public async Task ContentType_SetDisposed_ThrowsObjectDisposedException()
             Assert.Null(response.ContentType);
         }
 
-        // The managed implementation should not throw setting ContentType after the headers are sent.
-        [ConditionalFact(nameof(Helpers) + "." + nameof(Helpers.IsWindowsImplementation))] // [ActiveIssue(19971, TestPlatforms.AnyUnix)]
+        [ConditionalFact(nameof(PlatformDetection) + "." + nameof(PlatformDetection.IsNotOneCoreUAP))]
         public async Task ContentType_SetAfterHeadersSent_DoesNothing()
         {
             using (HttpListenerResponse response = await GetResponse())