Description:
------------
asXML() and saveXML() includes the XML declaration contrary to http://ca3.php.net/manual/en/libxml.constants.php

LIBXML_NOXMLDECL (integer)
Drop the XML declaration when saving a document

(#50989 is similar to this bug but was reported for DOMDocument.)

Test script:
---------------
<?php
$xml = new \SimpleXMLElement('<root/>', LIBXML_NOXMLDECL);
var_dump($xml->asXML());

Expected result:
----------------
string(8) "<root/>
"


Actual result:
--------------
string(30) "<?xml version="1.0"?>
<root/>
"