Fix for MVC post "preview draft" request

I have made a couple of adjustments 
 - 1 is a fix to the HintPath of the Piranha.csproj - a clean "clone" of the repo doesn't build
 - the other is a fix to the ability to preview a draft of a "Post" - right now 2.1.6 fails with a KeyNotFoundException when the preview of the post is requested (Piranha.Models.Post, line 264) - I tracked this back to the "IsDraft" property in BaseController - IsDraft isn't being assigned correctly when previewing a draft because the right-hand of the "Or" fails (BaseController.cs, line 39).   I fixed the controller type check to align with intent.