Description:
------------
ArrayIterator::seek() should (according to the manual: http://www.php.net/~helly/php/ext/spl/interfaceSeekableIterator.html)  "throw an exception if it is not possible to seek to the given position".

ArrayIterator::seek() does not. Instead, it sets the current element to NULL (as returned by ArrayIterator::current()).

Reproduce code:
---------------
$o = new ArrayIterator(array(1,2,3));
$o->seek(3);
var_dump($o->current());

Expected result:
----------------
Fatal error: Uncaught exception 'Exception' with message 'Could not seek to position...' ...

Actual result:
--------------
NULL