Example code to demo default IHttpResponseFeature throwing System.NotImplementedException

SampleTest.cs

using Xunit;
using MyNamespace;
using Microsoft.AspNetCore.TestHost;
using Microsoft.AspNetCore.Hosting.Server;
using Microsoft.AspNetCore.Hosting.Internal;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Http.Features;
using Microsoft.AspNetCore.Hosting;
using System.Threading.Tasks;

namespace MyNamespace.Tests
{
    public class SampleTest
    {
        [Fact]
        public async Task should_handle_request(){
            var testServer = new MyTestServer();
            var host = Startup.ConfigureHost(new WebHostBuilder()).UseServer(testServer).Build();
            host.Start();
            

            var httpContext = new DefaultHttpContext(){ RequestServices = host.Services };
            httpContext.Request.Path = "/not-found";

            await testServer.RequestHandler.Invoke(httpContext);
        }

    }

    class MyTestServer : IServer
    {
        RequestDelegate _requestHandler;

        public RequestDelegate RequestHandler { get { return _requestHandler; } }
        public IFeatureCollection Features { get; } = new FeatureCollection();
        public void Start<TContext>(IHttpApplication<TContext> application)
        {
            var webApplication = application as HostingApplication;
            _requestHandler = async httpContext => {
                var context = new HostingApplication.Context { HttpContext = httpContext };
                await webApplication.ProcessRequestAsync(context);
            };
        }

        public void Dispose()
        {
            _requestHandler = null;
        }
    }
}

Startup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using System.IO;
using Microsoft.AspNetCore.Http;

namespace MyNamespace
{
    public class Startup
    {
        public static void Main(string[] args)
        {
            var host = ConfigureHost(new WebHostBuilder()).Build();
            host.Run();
        }

        public static IWebHostBuilder ConfigureHost(IWebHostBuilder hostBuilder){
            return hostBuilder.UseContentRoot(Directory.GetCurrentDirectory())
                .UseKestrel()
                .UseStartup<Startup>();
        }

        // ConfigureServices is invoked before Configure
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddLogging();
            services.AddMvc();
            services.AddAuthorization();
        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
        {
            app.UseCookieAuthentication(new CookieAuthenticationOptions()
            {
                AuthenticationScheme = "Cookie",
                LoginPath = new PathString("/signin"),
                AccessDeniedPath = new PathString("/access-denied"),
                AutomaticAuthenticate = true,
                AutomaticChallenge = true
            });

            app.UseStaticFiles();
            app.UseMvc();
        }

    }
}