Description:
------------
XMLWriter differs in its handling of double-quotes when compared to other libxml
based PHP extensions.

Double-quotes using XMLWriter are converted to htmlentities (i.e. '"').
Using SimpleXML or DOMDocument double-quotes stay unconverted.

It seems to me that XMLWriter should not be converting double-quotes either.

Tidy seems to also agree -- try the following in a CLI:
$ echo "<?xml encoding=\"UTF-8\"?><tag>&quot;</tag>" | tidy -xml -q

Maybe introducing a flag/constant such as ENT_NOQUOTES to toggle the conversion of 
these on and off might be pertinent.

Test script:
---------------
<?php
$x = new XMLWriter;
$x->openMemory();
$x->startDocument('1.0', 'UTF-8');
$x->writeElement('tag', '"');
$x->endDocument();
echo $x->flush();


Expected result:
----------------
<?xml version=\"1.0\" encoding=\"UTF-8\"?><tag>"</tag>

Actual result:
--------------
<?xml version=\"1.0\" encoding=\"UTF-8\"?><tag>&quot;</tag>