Ensure we always have a headers array.

git-svn-id: https://develop.svn.wordpress.org/trunk@16063 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/class-IXR.php

@@ -603,6 +603,7 @@ class IXR_Client
     var $message = false;
     var $debug = false;
     var $timeout;
+    var $headers = array();
 
     // Storage place for an error message
     var $error = false;