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.
It's not meaningful to dispose of the HttpRequestMessage on the server. HRM.Dispose just calls HttpContent.Dispose, which on the client would close any upload streams. On the server side the server component will take care of the request stream cleanup.

If we were talking about disposing the HttpResponseMessage.Content that would be a different matter.