Fixed the copy function to look in the right place for a parent_link

Currently, if you try to copy a Page while using a OneToOneField of
any kind, it will fail.

wagtail/wagtailcore/models.py

@@ -1079,7 +1079,7 @@ class Page(six.with_metaclass(PageBase, MP_Node, ClusterableModel, index.Indexed
                 continue
 
             # Ignore parent links (page_ptr)
-            if isinstance(field, models.OneToOneField) and field.parent_link:
+            if isinstance(field, models.OneToOneField) and field.rel.parent_link:
                 continue
 
             specific_dict[field.name] = getattr(specific_self, field.name)