Using Action.async with Rendering.render requires downcast

The Rendering trait has not been updated to reflect recent API changes. Specifically, the "render" method returns a Result, which has been deprecated in favor of SimpleResult. This has the unfortunate effect of requiring downcast when used with Action.async, which expects a function that returns a Future[SimpleResult]. 

As far as I can tell, fixing this should just be a matter of changing the return type of Rendering.render.