Allow per-request controller_path.

Don't let the description fool you -- it's a trivial, two-line patch.

This is about how ActionController::Base#controller_path is used.

From the commit summary:

The instance method calls the class method by default. With this change, controllers can override the instance method to set a dynamic controller_path for a given request.

Rationale:

It is possible to override the whole view_path (with prepend_view_path and append_view_path) for all requests, or for an individual request. It seems that the controller_path is a smaller change.

In fact, I believe that any argument against doing this would apply even moreso to modifying the view_path.