Add wp_json_encode(), a wrapper for json_encode() that ensures everything is converted to UTF-8.

Change all core calls from `json_encode()` to `wp_json_encode()`.

Fixes #28786.


Built from https://develop.svn.wordpress.org/trunk@30055


git-svn-id: http://core.svn.wordpress.org/trunk@30055 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/media.php

@@ -1826,7 +1826,7 @@ if( !$large_size_w )
 	$large_size_w = 1024;
 ?>
 var resize_height = <?php echo $large_size_h; ?>, resize_width = <?php echo $large_size_w; ?>,
-wpUploaderInit = <?php echo json_encode($plupload_init); ?>;
+wpUploaderInit = <?php echo wp_json_encode( $plupload_init ); ?>;
 </script>
 
 <div id="plupload-upload-ui" class="hide-if-no-js">