Description:
------------
the simplexmlelement can not currently modify cdata node


Reproduce code:
---------------
<?php
$xml_string = <<<EOD
<?xml version="1.0" encoding="iso-8859-1" standalone="yes" ?>
<a>
 <b>b</b>
 <c><![CDATA[c]]></c>
</a>
EOD;

$xml = simplexml_load_string($xml_string);
$a = $xml->xpath('//a');
$a[0]-> b = B;
$a[0]-> c = C;

echo $xml-> saveXML();
?>

<a>
 <b>B</b>
 <c>C</c>
</a>

Expected result:
----------------
<?php
$xml_string = <<<EOD
<?xml version="1.0" encoding="iso-8859-1" standalone="yes" ?>
<a>
 <b>b</b>
 <c><![CDATA[c]]></c>
</a>
EOD;

$xml = simplexml_load_string($xml_string);
$a = $xml->xpath('//a');
$a[0]-> b = B;
$a[0]-> c = C;

echo $xml-> saveXML();
?>

<a>
 <b>B</b>
 <c><![CDATA[C]]</c>
</a>