code: def set_def_canned_acl(self, acl_str, key_name='', headers=None):
          
          return self.set_canned_acl_helper(acl_str, key_name, headers, 
                                            query_args=DEF_OBJ_ACL)
docs: sets or changes a bucket's default object acl to a predefined 
             (canned) value