Fix method HttpHeadersNonValidated uses to get HeaderDescriptor

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeaders.cs

@@ -1051,7 +1051,7 @@ private HeaderDescriptor GetHeaderDescriptor(string name)
             throw new InvalidOperationException(SR.Format(SR.net_http_headers_not_allowed_header_name, name));
         }
 
-        private bool TryGetHeaderDescriptor(string name, out HeaderDescriptor descriptor)
+        internal bool TryGetHeaderDescriptor(string name, out HeaderDescriptor descriptor)
         {
             if (string.IsNullOrEmpty(name))
             {

src/libraries/System.Net.Http/src/System/Net/Http/Headers/HttpHeadersNonValidated.cs

@@ -32,7 +32,7 @@ namespace System.Net.Http.Headers
         /// <returns>true if the collection contains the header; otherwise, false.</returns>
         public bool Contains(string headerName) =>
             _headers is HttpHeaders headers &&
-            HeaderDescriptor.TryGet(headerName, out HeaderDescriptor descriptor) &&
+            headers.TryGetHeaderDescriptor(headerName, out HeaderDescriptor descriptor) &&
             headers.TryGetHeaderValue(descriptor, out _);
 
         /// <summary>Gets the values for the specified header name.</summary>
@@ -62,7 +62,7 @@ public HeaderStringValues this[string headerName]
         public bool TryGetValues(string headerName, out HeaderStringValues values)
         {
             if (_headers is HttpHeaders headers &&
-                HeaderDescriptor.TryGet(headerName, out HeaderDescriptor descriptor) &&
+                headers.TryGetHeaderDescriptor(headerName, out HeaderDescriptor descriptor) &&
                 headers.TryGetHeaderValue(descriptor, out object? info))
             {
                 HttpHeaders.GetStoreValuesAsStringOrStringArray(descriptor, info, out string? singleValue, out string[]? multiValue);

src/libraries/System.Net.Http/tests/UnitTests/Headers/HttpHeadersTest.cs

@@ -2112,6 +2112,44 @@ public void AddHeaders_SourceHasInvalidHeaderValues_InvalidHeadersRemovedFromSou
             Assert.False(destination.Contains("custom"), "destination contains 'custom' header.");
         }
 
+        [Fact]
+        public void AddHeaders_ResponseHeaderToRequestHeaders_Success()
+        {
+            const string Name = "WWW-Authenticate";
+            const string Value = "Basic realm=\"Access to the staging site\", charset=\"UTF-8\"";
+
+            var request = new HttpRequestMessage();
+            Assert.True(request.Headers.TryAddWithoutValidation(Name, Value));
+
+            Assert.True(request.Headers.Contains(Name));
+            Assert.True(request.Headers.NonValidated.Contains(Name));
+
+            Assert.True(request.Headers.TryGetValues(Name, out IEnumerable<string> values));
+            Assert.Equal(Value, values.Single());
+
+            Assert.True(request.Headers.NonValidated.TryGetValues(Name, out HeaderStringValues nvValues));
+            Assert.Equal(Value, nvValues.Single());
+        }
+
+        [Fact]
+        public void AddHeaders_RequestHeaderToResponseHeaders_Success()
+        {
+            const string Name = "Referer";
+            const string Value = "https://dot.net";
+
+            var response = new HttpResponseMessage();
+            Assert.True(response.Headers.TryAddWithoutValidation(Name, Value));
+
+            Assert.True(response.Headers.Contains(Name));
+            Assert.True(response.Headers.NonValidated.Contains(Name));
+
+            Assert.True(response.Headers.TryGetValues(Name, out IEnumerable<string> values));
+            Assert.Equal(Value, values.Single());
+
+            Assert.True(response.Headers.NonValidated.TryGetValues(Name, out HeaderStringValues nvValues));
+            Assert.Equal(Value, nvValues.Single());
+        }
+
         [Fact]
         public void HeaderStringValues_Default_Empty()
         {