Description:
------------
mysqli_insert_id return string when the result greater than the max_value of signed int. But the document http://www.php.net/manual/en/mysqli.insert-id.php says it return int, not mention the possibility of returning string.

Reproduce code:
---------------
CREATE TABLE `bigint_test` (
  `id` bigint(20) NOT NULL auto_increment,
  `vv` varbinary(100) default NULL,
  PRIMARY KEY  (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=8589934592;
$l->query('INSERT INTO bigint_test (`vv`) values ("ii")');
var_dump($l->insert_id);

Expected result:
----------------
mixed mysqli_insert_id  ( mysqli $link  )

Actual result:
--------------
int mysqli_insert_id  ( mysqli $link  )