hook to the 'posts_selection' action to restore the $wp_query->is_author flag

set it back to `TRUE` if we have a coauthors-plus taxonomy query on our hands

see https://github.com/GigaOM/gigaom/issues/4904
Perhaps worth adding a code comment explaining why:

> this method set `$wp_query->is_author` to `FALSE`, so we need to go back later to set it `TRUE` again for compatibility with other code that expects `is_author()` to be `TRUE` for author queries