Unexpected NTLM message Type 3 - should respond with 401

When express-ntlm receives an unexpected NTLM message Type 3 it responds with 500 (internal server error). It would be more helpful to the client if it responds with 401 (unauthorized), since this will trigger a proper client to re-authenticate. 
I have tested this scenario against a Microsoft IIS, and it responds with a 401.

Rather simple change, but I will provide a PR for it nonetheless :)