Description:
------------
Using $xml->asXML() to output an XML document as a string from SimpleXML seems to be defaulting to ISO 8859-1 rather than UTF-8, despite all other operations being in UTF-8 (and with LANG and LC_ALL being set to UTF-8).

There is a workaround, by manually setting '<?xml version="1.0" encoding="UTF-8"?>' at the start of any imported XML, but it seems strange that there isn't anywhere to set this default permanently.

The behaviour of asXML() also seems to vary when printing part of a SimpleXML object (where it uses UTF-8) rather than the whole document (where it uses ISO 8859-1).

Adding
putenv('LANG=en_GB.UTF-8');
setlocale(LC_ALL, 'en_GB.UTF-8');
to the script doesn't seem to help.

Reproduce code:
---------------
// manually set encoding to UTF-8
$doc = simplexml_load_string('<?xml version="1.0" encoding="UTF-8"?><text>umlaut ? here</text>');
print $doc->asXML() . "\n";

// defaults to UTF-8
$doc = simplexml_load_string('<doc><text>umlaut ? here</text></doc>');
print $doc->text->asXML() . "\n\n";

// defaults to ISO 8859-1
$doc = simplexml_load_string('<text>umlaut ? here</text>');
print $doc->asXML() . "\n";

Expected result:
----------------
<?xml version="1.0" encoding="UTF-8"?>
<text>umlaut ? here</text>

<text>umlaut ? here</text>

<?xml version="1.0"?>
<text>umlaut ? here</text>



Actual result:
--------------
<?xml version="1.0" encoding="UTF-8"?>
<text>umlaut ? here</text>

<text>umlaut ? here</text>

<?xml version="1.0"?>
<text>umlaut &#xFC; here</text>