Display Proper Author Information For Posts != $post->ID

When get_coauthors() is called with a post ID (`$post_id`) specified, `$post->ID`'s author is displayed if `$post_id` is created by a "real" user (non-coauthor), even if `$post_id != $post->ID`.

This corrects this functionality, and will display the current post's author if and only if `$post_id == $post->ID`.