Description:
------------
I use xmlWriter a lot, but I found a quite inconsistent behavior with booleans in 
xmlWriter. 

When filling an element/namespace with a boolean : 

- if the boolean is true, it is automatically casted to an int (1), which is a 
legal representation of the boolean. 
- if the boolean is false, no value is given to the element/attribute. 

I made a little script to reproduce the problem.It runs well in CLI

Test script:
---------------
<?php

xml(true);
xml(false);

function xml($bool) {
	$xml = new XmlWriter();
	$xml->openMemory();
	$xml->setIndent(true);
	$xml->startElement('root');
	$xml->writeAttribute('boolAttr', $bool);
	$res = $xml->writeElement('myboolean',$bool);
	$xml->endElement();
	echo $xml->flush().PHP_EOL;
	var_dump($res);
}

Expected result:
----------------
<root boolAttr="1">
 <myboolean>1</myboolean>
</root>

bool(true)
<root boolAttr="0">
 <myboolean>0</myboolean>
</root>

Actual result:
--------------
<root boolAttr="1">
 <myboolean>1</myboolean>
</root>

bool(true)
<root boolAttr="">
 <myboolean></myboolean>
</root>