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 there explaining that it's added conditionally in `$this->parse_query()`and we only want to run it once after adding it.