Description:
------------
SimpleXMLElement->addChild() causes warning message and produces 
incomplete XML in case when the second argument contains ampersand  
sign '&'. Text after ampersand is absent in output XML.

In the same time escaping of other XML special symbols in tag body 
works OK. Also all XML special symbols in tag attributes are escaped 
OK.


Reproduce code:
---------------
<?
$sxml=new SimpleXMLElement('<?xml version="1.0" encoding="utf-8"?><test></test>');
// Quotes
$sitem=$sxml->addChild('tag',"'");
$sitem->addAttribute('attr',"'");
$sitem=$sxml->addChild('tag','"');
$sitem->addAttribute('attr','"');
// Brackets
$sitem=$sxml->addChild('tag','<');
$sitem->addAttribute('attr','<');
$sitem=$sxml->addChild('tag','>');
$sitem->addAttribute('attr','>');
// Ampersand
$sitem=$sxml->addChild('tag','Smith & Wesson');
$sitem->addAttribute('attr','Smith & Wesson');
// Result XML
print($sxml->asXML());
?>


Expected result:
----------------
<?xml version="1.0" encoding="utf-8"?>
<test><tag attr="'">'</tag><tag attr="&quot;">"</tag><tag 
attr="&lt;">&lt;</tag><tag attr="&gt;">&gt;</tag><tag attr="Smith 
&amp; Wesson">Smith &amp; Wesson</tag></test>


Actual result:
--------------
PHP Warning:  SimpleXMLElement::addChild(): unterminated entity 
reference          Wesson in xml.php on line 14

Warning: SimpleXMLElement::addChild(): unterminated entity reference          
Wesson in xml.php on line 14
<?xml version="1.0" encoding="utf-8"?>
<test><tag attr="'">'</tag><tag attr="&quot;">"</tag><tag 
attr="&lt;">&lt;</tag><tag attr="&gt;">&gt;</tag><tag attr="Smith 
&amp; Wesson">Smith </tag></test>