isset($_GET["preview"]) has been added

git-svn-id: https://plugins.svn.wordpress.org/wp-fastest-cache/trunk@772705 b8457f37-d9ea-0310-8a92-e5e31aec5664
isset($_GET["preview"]) has been added

git-svn-id: https://plugins.svn.wordpress.org/wp-fastest-cache/trunk@772706 b8457f37-d9ea-0310-8a92-e5e31aec5664