Commented Issue: Make MVC UrlHelper methods virtual [1425]

UrlHelper methods in web API are virtual and are of great use while writing unit tests. Pass on the goodness to MVC UrlHelper as well.
Comments: [Fixed](https://aspnetwebstack.codeplex.com/SourceControl/changeset/fa0e89021087558c49f3f90cdd6ee9d7325fe805)