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.
You could consider moving this to a separate method so that the property is easier to read
```C#
if (_httpRequestMessage == null)
{
   _httpRequestMessage = CreateHttpRequestMessage();
}
return _httpRequestMessage;
```