Fix ProxyExplicitlyProvided_DefaultCredentials_Ignored test (#28770)

I couldn't get it to fail locally, but I believe the issue was that sporadically WinHttpHandler would not reuse the original connection, in which case it would either not close the first and the test would hang because nothing was accepting the second connection, or it would close the first and fail because the server was expecting a second request on the connection.  This fixes it by using Connection: close on the first response so that the client won't reuse the same connection.

Author: stephentoub

src/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.DefaultProxyCredentials.cs

@@ -50,41 +50,30 @@ public async Task ProxyExplicitlyProvided_DefaultCredentials_Ignored()
         {
             var explicitProxyCreds = new NetworkCredential("rightusername", "rightpassword");
             var defaultSystemProxyCreds = new NetworkCredential("wrongusername", "wrongpassword");
-            string expectCreds = "Basic " + Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes($"{explicitProxyCreds.UserName}:{explicitProxyCreds.Password}"));
+            string expectCreds = "Basic " + Convert.ToBase64String(Encoding.UTF8.GetBytes($"{explicitProxyCreds.UserName}:{explicitProxyCreds.Password}"));
 
-            await LoopbackServer.CreateServerAsync(async (proxyServer, proxyUrl) =>
+            await LoopbackServer.CreateClientAndServerAsync(async proxyUrl =>
             {
                 using (HttpClientHandler handler = CreateHttpClientHandler())
                 using (var client = new HttpClient(handler))
                 {
                     handler.Proxy = new UseSpecifiedUriWebProxy(proxyUrl, explicitProxyCreds);
                     handler.DefaultProxyCredentials = defaultSystemProxyCreds;
-
-                    // URL does not matter. We will get response from "proxy" code bellow.
-                    Task<HttpResponseMessage> responseTask = client.GetAsync("http://notatrealserver.com/");
-
-                    await proxyServer.AcceptConnectionAsync(async connection =>
+                    using (HttpResponseMessage response = await client.GetAsync("http://notatrealserver.com/")) // URL does not matter
                     {
-                        List<string> headers = await connection.ReadRequestHeaderAsync();
-
-                        if (!IsCurlHandler)
-                        {
-                            // Curl sends Basic auth without asking, other handlers wait for 407.
-                            await connection.SendResponseAsync(HttpStatusCode.ProxyAuthenticationRequired, "Proxy-Authenticate: Basic\r\n");
-                            headers = await connection.ReadRequestHeaderAsync();
-                        }
-
-                        // Verify that we got explicitProxyCreds.
-                        Assert.Equal(expectCreds, LoopbackServer.GetRequestHeaderValue(headers, "Proxy-Authorization"));
-
-                        Task serverTask = connection.SendResponseAsync(HttpStatusCode.OK);
-
-                        await TestHelper.WhenAllCompletedOrAnyFailed(serverTask, responseTask);
-                        HttpResponseMessage response = responseTask.Result;
-
                         Assert.Equal(response.StatusCode, HttpStatusCode.OK);
-                    });
-                };
+                    }
+                }
+            }, async server =>
+            {
+                if (!IsCurlHandler) // libcurl sends Basic auth preemptively when only basic creds are provided; other handlers wait for 407.
+                {
+                    await server.AcceptConnectionSendResponseAndCloseAsync(
+                        HttpStatusCode.ProxyAuthenticationRequired, "Connection: close\r\nProxy-Authenticate: Basic\r\n");
+                }
+
+                List<string> headers = await server.AcceptConnectionSendResponseAndCloseAsync(HttpStatusCode.OK);
+                Assert.Equal(expectCreds, LoopbackServer.GetRequestHeaderValue(headers, "Proxy-Authorization"));
             });
         }