add filter to wp_dropdown_users() in post_author_meta_box

[jkudish]

In a use case where I don't want to show "authors" only in the author dropdown but want to be able to assign a post to any registered user, there is no way currently to filter the author's meta box. The proposed patch adds a filter on $who variable of the dropdown.

*the $who, typo :)

I would unregister the meta box and re-register it with your own wp_dropdown_users() call.

For a filter, I'd like to see something on the entire array, rather than just the 'who' value. But switching out the meta box (and using the same save code) is a simple workaround.

Patch which filters the whole array

removed _ref_array() and added (array) cast.