PHP5.5 fix: Better use of startXML for Action classes (mostly AJAX)

I had a problem with PHP5.5 that caused ajax responses to be empty. This
fixes it, as the problem was related to pretty inconsistent calling to
headers, XMLWriter::startDocument etc. etc.

plugins/QnA/actions/qnanewanswer.php

@@ -151,10 +151,8 @@ class QnanewanswerAction extends Action
         if ($this->boolean('ajax')) {
             common_debug("ajaxy part");
             $answer = $this->question->getAnswer($profile);
-            header('Content-Type: text/xml;charset=utf-8');
-            $this->xw->startDocument('1.0', 'UTF-8');
 
-            $this->elementStart('html');
+            $this->startHTML('text/xml;charset=utf-8');
             $this->elementStart('head');
             // TRANS: Page title after sending an answer.
             $this->element('title', null, _m('Answers'));
@@ -166,7 +164,7 @@ class QnanewanswerAction extends Action
             $nli->show();
 
             $this->elementEnd('body');
-            $this->elementEnd('html');
+            $this->endHTML();
         } else {
             common_debug("not ajax");
             common_redirect($this->question->bestUrl(), 303);