Description:
------------
It seems that Indentation is not working well when the XMLWriter::writePi() method is called.

Reproduce code:
---------------
<?php 
$X = new XMLWriter;
$X->openMemory();
$X->setIndent(true);
$X->startDocument('1.0', 'UTF-8');
$X->writePi('xml-stylesheet' , 'type="text/css" href="/style.css"');
$X->writePi('xml-stylesheet' , 'type="text/xsl" href="/style.xsl"');
$X->writeComment('XMLWriter - PHP5.2');
$X->startElement ('tag');
$X->endElement();
echo $X->flush();
?>

Expected result:
----------------
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/css" href="/mon-style.css"?>
<?xml-stylesheet type="text/xsl" href="/mon-style.xsl"?>
<!--XMLWriter - PHP5.2-->
<tag/>

Actual result:
--------------
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/css" href="/mon-style.css"?><?xml-stylesheet type="text/xsl" href="/mon-style.xsl"?><!--XMLWriter - PHP5.2-->
<tag/>