interface.register_action: use Roby::Task as default return type

The new generated task model cannot be used as default return type
because it cannot be replaced by other tasks derevied from Roby::Tasks

task_model.has_ancestor?(MyTask) ==> false

lib/roby/actions/models/interface.rb

@@ -91,7 +91,7 @@ def register_action(name, action_model)
                         end
                         const_set task_model_name, task_model
                         task_model.permanent_model = self.permanent_model?
-                        action_model.returns(task_model)
+                        action_model.returns(task_model.superclass)
                     end
                 end