use nested_set_scope when selecting and locking the rows in the model…

… that extend past the deletion point

lib/awesome_nested_set/awesome_nested_set.rb

@@ -535,7 +535,7 @@ def destroy_descendants
           in_tenacious_transaction do
             reload_nested_set
             # select the rows in the model that extend past the deletion point and apply a lock
-            self.class.base_class.where(["#{quoted_left_column_name} >= ?", left]).
+            nested_set_scope.where(["#{quoted_left_column_name} >= ?", left]).
                                   select(id).lock(true)
 
             if acts_as_nested_set_options[:dependent] == :destroy