Custom header: Fix image ordering.

props ehg.
see #21785. fixes #27791.


git-svn-id: https://develop.svn.wordpress.org/trunk@28102 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/js/customize-controls.js

@@ -482,6 +482,7 @@
 
 			data.url = url;
 			data.thumbnail_url = url;
+			data.timestamp = _.now();
 
 			if (attachmentId) {
 				data.attachment_id = attachmentId;