Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 16828)
+++ wp-includes/functions.php	(working copy)
@@ -4220,10 +4220,8 @@
 	if ( $context != '' ) {
 		$extra_headers = apply_filters( "extra_{$context}_headers", array() );
 
-		$extra_headers = array_flip( $extra_headers );
-		foreach( $extra_headers as $key=>$value ) {
-			$extra_headers[$key] = $key;
-		}
+		$extra_headers = array_combine( $extra_headers, $extra_headers );
+
 		$all_headers = array_merge( $extra_headers, (array) $default_headers );
 	} else {
 		$all_headers = $default_headers;