Fix: URL-encode the selected_actor query parameter. 

A user reported that the unencoded ":" character in the query string was causing errors. I was previously operating under the mistaken assumption that add_query_arg() would URL-encode the arguments. It does not; plugin code has to take care of it.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@920637 b8457f37-d9ea-0310-8a92-e5e31aec5664