#52 EditArea breaks with application/xhtml+xml

Consider the PHP script attached. When using it as it is now, things seem to work properly. However, uncommenting the header() call (which will replace the default Content-Type "text/html" with "application/xhtml+xml") leads to these consequences:
1. submit_callback is no more being called;
2. editing the text and pressing "Submit" sends the original, not the modified, text to the server (however, if one clicks "Toggle editor" at least once after editing, the new text is submitted).

This behavior is observable and identical in Mozilla Firefox 3, Opera and Google Chrome.

[Anonymous]

Test case