Better check to see if the XML prolog should be outputted for XML

documents i.e., if best mimetype is */*xml, then use the XML prolog.

lib/htmloutputter.php

@@ -109,8 +109,8 @@ class HTMLOutputter extends XMLOutputter
         header('Content-Type: '.$type.'; charset=UTF-8');
 
         $this->extraHeaders();
-        if( ! substr($type,0,strlen('text/html'))=='text/html' ){
-            // Browsers don't like it when <?xml it output for non-xhtml documents
+        if (preg_match("/.*\/.*xml/", $type)) {
+            // Required for XML documents
             $this->xw->startDocument('1.0', 'UTF-8');
         }
         $this->xw->writeDTD('html',