Refactor Sprockets::Helpers::RailsHelper#performing_caching?

No need for them lines to get so long! Also, move that comment into
the method body where it belongs!

actionpack/lib/sprockets/helpers/rails_helper.rb

@@ -139,9 +139,14 @@ def rewrite_extension(source, dir, ext)
           end
         end
 
-        # When included in Sprockets::Context, we need to ask the top-level config as the controller is not available
         def performing_caching?
-          config.action_controller.present? ? config.action_controller.perform_caching : config.perform_caching
+          # When included in Sprockets::Context, we need to ask the
+          # top-level config as the controller is not available.
+          if config.action_controller.present?
+            config.action_controller.perform_caching
+          else
+            config.perform_caching
+          end
         end
       end
     end