Fix NullReference when using T4MVC

T4MVC tries using the TwilioResponse constructor for the TwiMLResult object when generating the URL using URL.Action which causes a Runtime Exception.

More details can be found at https://t4mvc.codeplex.com/workitem/22
Merge pull request #72 from MatthewSteeples/master

Fix for exception when using Twilio.Mvc with T4MVC