Description:
------------
If you add elements with addChild() and then call asXML() the added elements are printed on the same line. This makes the XML difficult to read.

Reproduce code:
---------------
<?php
        $xmlstr = <<<XML
<a>
        <b>bbb</b>
</a>
XML;
        $xml = new SimpleXMLElement($xmlstr);
        $xml->addChild('c', 'ccc');
        $xml->addChild('d', 'ddd');
        echo $xml->asXML();
?>


Expected result:
----------------
<?xml version="1.0"?>
<a>
        <b>bbb</b>
        <c>ccc</c>
        <d>ddd</d>
</a>



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