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
I'm assuming based on this line that we _only_ want this action executing for the _first_ `get_posts()` we encounter (the location of the associated `do_action`).  What about manually executed `get_posts()` for authors?  Just curious why this is a one-time execution.