Index: wp-includes/class-IXR.php
===================================================================
--- wp-includes/class-IXR.php	(revision 10562)
+++ wp-includes/class-IXR.php	(working copy)
@@ -301,6 +301,7 @@
         if (!$data) {
             global $HTTP_RAW_POST_DATA;
             if (!$HTTP_RAW_POST_DATA) {
+               header( 'Content-Type: text/plain' );
                die('XML-RPC server accepts POST requests only.');
             }
             $data = $HTTP_RAW_POST_DATA;