sets or changes a bucket's default object acl to a predefined 
           (canned) value

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