Return a single post type, not an array

[koyan]

Fixes error happening on prospect when trying to render menu item. The current code returns a whole array, while it should be returning a string. It was introduced at https://github.com/outlandishideas/oowp/pull/20 Before it was returning the word 'any'

[rasmuswinter]

this change doesn't solve the problem, as now it will arbitrarily choose the first valid post type. instead, it should exclude 'menu_item' (or whatever post type you're getting) as well as 'attachment'

[koyan]

Fixed