Description:
------------
An XML-String appended to a DOMDocumentFragment with
appendXML() must always be encoded with UTF-8, regardless
of the choosen DOMDocument encoding. Even worse, after
appending the fragment to the document, the charset in
the fragment remains UTF-8, which leads to a DOMDocument
with mixed encodings inside.

Reproduce code:
---------------
$dom = new DOMDocument('1.0','ISO-8859-1');
$dom->loadXML('<parent />');
$frag = $dom->createDocumentFragment();
$frag->appendXML('<child>???</child>');
$dom->documentElement->appendChild($frag);


Expected result:
----------------
the ISO-8859-1 encoded XML-String will cleanly import into
the document fragment.

Actual result:
--------------
Error:

DOMDocumentFragment::appendXML() Entity: line 1: parser error : Input is not proper UTF-8, indicate encoding !!

When trying to use an XML-declaration containing the ISO-8859-1 encoding in the child-XML-String:

Entity: line 1: parser error : XML declaration allowed only at the start of the document

When you convert the child-XML-String to utf8 and append it, you have a DOMDocument with mixed charset