Getting actual user object for author meta when coauthors-plus is active

Part 1 of a solution for:
GigaOM/gigaom#4904

See also:
https://github.com/GigaOM/gigaom-plugins/pull/2879
I can see how this fixes it, but it adds specific and fragile awareness of `coauthors-plus` to a separate plugin.

Perhaps the better solution is to do an `apply_filters( 'the_queried_object', get_queried_object() )` or similar here, and in `go-local-coauthors-plus` we can replace it with the right stuff.