Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 13925)
+++ wp-includes/theme.php	(working copy)
@@ -1357,7 +1357,7 @@
 function register_default_headers( $headers ) {
 	global $_wp_default_headers;
 
-	$_wp_default_headers = $headers;
+	$_wp_default_headers = array_merge( (array)$_wp_default_headers, (array) $headers);
 }
 
 /**