Fix for issue #1279 - Add .Request property to ApiController

This change adds a .Request property to the ApiController class that can
be used to access an HttpRequestMessage wrapping the HttpContext.

The HttpRequestMessage is stored in an http feature to make it accessible
to model binders and other infrastructure.
Per conversation with @Tratcher we're going to just leave it alone right now (no dispose). If we need to bring back the legacy `RegisterForDispose` feature back we'll consider costing of building support into hosting.