#19 Serializer inconsitant use of " and '

[Patrick Tuin]

The serializer writes out
<?xml version='1.0' encoding='UTF-8' standalone='yes' ?> as a header (using ')
while the rest of the xml attributes are written out between "
<?xml-stylesheet href="some_stylesheet.xslt" type="text/xsl"?>

serializer-attvalue-use-apostrophe is not implemented