Merge pull request #2516 from jchannon/responsecastfix

Check routeresult is Response

src/Nancy/Responses/Negotiation/DefaultResponseNegotiator.cs

@@ -80,6 +80,12 @@ private static bool TryCastResultToResponse(dynamic routeResult, out Response re
             var targetType = routeResult.GetType();
             var responseType = typeof(Response);
 
+            if (routeResult is Response)
+            {
+                response = (Response)routeResult;
+                return true; 
+            }
+
             var methods = responseType.GetMethods(BindingFlags.Public | BindingFlags.Static);
 
             foreach (var method in methods)