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 think it's because the action is only added (line 124 above) if and only if the WP_Query was successfully converted from an author query to a taxonomy query by `go-sphinx`. I was worried that if I leave it on all the time then it might do something to a WP_Query that should not be converted back to an author query. Is not a good thing to do?