Fixed :scheme header for H/2 with PlaintextStreamFilter (#71057)

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Connection.cs

@@ -1424,7 +1424,7 @@ private void WriteHeaders(HttpRequestMessage request, ref ArrayBuffer headerBuff
                 WriteIndexedHeader(H2StaticTable.MethodGet, normalizedMethod.Method, ref headerBuffer);
             }
 
-            WriteIndexedHeader(_stream is SslStream ? H2StaticTable.SchemeHttps : H2StaticTable.SchemeHttp, ref headerBuffer);
+            WriteIndexedHeader(_pool.IsSecure ? H2StaticTable.SchemeHttps : H2StaticTable.SchemeHttp, ref headerBuffer);
 
             if (request.HasHeaders && request.Headers.Host != null)
             {

src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs

@@ -3193,7 +3193,12 @@ public async Task PlaintextStreamFilter_SimpleDelegatingStream_Success(bool useS
                 },
                 async server =>
                 {
-                    await server.AcceptConnectionSendResponseAndCloseAsync(content: "foo");
+                    HttpRequestData request = await server.AcceptConnectionSendResponseAndCloseAsync(content: "foo");
+                    if (request.Version == HttpVersion20.Value)
+                    {
+                        HttpHeaderData schemeHeader = Assert.Single(request.Headers, headerData => headerData.Name == ":scheme");
+                        Assert.Equal(useSsl ? "https" : "http", schemeHeader.Value);
+                    }
                 }, options: options);
         }