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
Rather than testing all this stuff, perhaps just set an object var in `$this->parse_query()` and use that to determine if you should proceed? That would save duplicate testing.