Fix #812 - allow to return a struct as a model for a route

Just need to check if return is a value type before testing against null. Fix #812.
Then, you can use embedded JsonSerializer or JSON.NET one without any problem.
Thanks
I receive some mails with inline comments here from @jchannon, but I don't see them anymore...
You are right result is not used, I used the same test as the one I rename (Should_invoke_response_negotiator_for_reference_model), that's why...I can remove it.

@horsdal is right, if you remove my fix in DefaultRouteInvoker, you will see that this test fails with the error I described in #812 (so responseNegotiator is not called)