Better handling of httpcontext in SampleIdentityServer3

- HttpContext.Current is not reliable in all scenarious, grabbing it from
  the owin dictionary instead.
- Ignore headerswritten check if no httpcontext is found to make the same
  code work in self hosted environments too.

SampleIdentityServer3/Startup.cs

@@ -155,9 +155,11 @@ private void UseIdSrv3LogoutOnFederatedLogout(IAppBuilder app, KentorAuthService
             {
                 await next.Invoke();
 
+                var httpContext = context.Get<HttpContextBase>(typeof(HttpContextBase).FullName);
+
                 if (context.Authentication.AuthenticationResponseRevoke != null
                     && context.Response.StatusCode % 100 == 3
-                    && !HttpContext.Current.Response.HeadersWritten)
+                    && (httpContext == null || !httpContext.Response.HeadersWritten))
                 {
                     var finalLocation = context.Response.Headers["Location"];