PR fixes

src/Microsoft.AspNet.Authentication/AuthenticationHandler.cs

@@ -113,10 +113,16 @@ namespace Microsoft.AspNet.Authentication
         /// </summary>
         /// <returns>async completion</returns>
         internal async Task TeardownAsync()
+        {
+            try
             {
                 await FinishResponseOnce();
+            }
+            finally
+            {
                 UnregisterAuthenticationHandler();
             }
+        }
 
         /// <summary>
         /// Called once by common code after initialization. If an authentication middleware responds directly to