Ignore MaxRequestBodySize for upgraded requests (#13477)

src/Servers/IIS/IIS/src/Core/IISHttpContext.FeatureCollection.cs

@@ -312,6 +312,7 @@ namespace Microsoft.AspNetCore.Server.IIS.Core
                 throw new InvalidOperationException(CoreStrings.UpgradeCannotBeCalledMultipleTimes);
             }
 
+            MaxRequestBodySize = null;
             _wasUpgraded = true;
 
             StatusCode = StatusCodes.Status101SwitchingProtocols;

src/Servers/IIS/IIS/test/testassets/InProcessWebSite/Startup.WebSockets.cs

@@ -98,6 +98,7 @@ namespace TestSite
 
             // Upgrade the connection
             Stream opaqueTransport = await upgradeFeature.UpgradeAsync();
+            Assert.Null(context.Features.Get<IHttpMaxRequestBodySizeFeature>().MaxRequestBodySize);
 
             // Get the WebSocket object
             var ws = WebSocket.CreateFromStream(opaqueTransport, isServer: true, subProtocol: null, keepAliveInterval: TimeSpan.FromMinutes(2));