Description:
------------
When changing child's in cloned object, it's change in parent object, not in 
cloned.

Test script:
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$xml = '<a><b></b></a>';

$o1 = new SimpleXMlElement($xml);
$o2 = clone $o1;

$r = current($o2->xpath('/a'));
$r->addChild('c',new SimpleXMlElement('<c></c>'));

echo $o1->asXML();
echo PHP_EOL;
echo $o2->asXML();

Expected result:
----------------
<?xml version="1.0"?>
<a><b/></a>

<a><b/><c></c></a>

Actual result:
--------------
<?xml version="1.0"?>
<a><b/><c></c></a>

<a><b/></a>