Description:
------------
If an ArrayIterator was appended to an AppendIterator, and the inner pointer of ArrayIterator was changed (foreach, ->next or something else), it would also effect AppendIterator in a strange way, see the test script (https://3v4l.org/EavLv) below. 

I'm not sure if the iterators were appended by reference or value (clone a new copy), but current result cannot be explained in either way. 

Test script:
---------------
<?php

$iterator = new \AppendIterator();
$events = new \ArrayIterator([1,2,3,4,5]);
$iterator->append($events);

$events->next();

while($iterator->valid()) {
	echo $iterator->current(), "\n";
	$iterator->next();
}

Expected result:
----------------
append reference:
2
3
4
5

or
append clone:
1
2
3
4
5

Actual result:
--------------
1
3
4
5