Set the correct header for non POST request to xmlrpc service. Fixes #9093 props joesphscott.

git-svn-id: https://develop.svn.wordpress.org/trunk@10571 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/class-IXR.php

@@ -301,6 +301,7 @@ class IXR_Server {
         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;