also consider "author" taxonomy query when testing whether we should add author opengraph metadata

I think we need @misterbisson's eyes on this one.

see https://github.com/GigaOM/gigaom/issues/4904
our go-local-coauthors-plus explicitly [turns off the `is_author()` condition](https://github.com/GigaOM/go-local-coauthors-plus/blob/a6269a6244a02b0f07cd0ac9976b2e5232b699ca/components/class-go-local-coauthors-plus-query.php#L119) after it converts an author query into an author taxonomy query.

unfortunately WP doesn't call a filter to allow us to modify the results of the [`is_author()` function](https://codex.wordpress.org/Function_Reference/is_author), and if we want that function to pass at all then we must not set `$wp_query`'s `is_author` flag to `FALSE` like we do [here](https://github.com/GigaOM/go-local-coauthors-plus/blob/a6269a6244a02b0f07cd0ac9976b2e5232b699ca/components/class-go-local-coauthors-plus-query.php#L119) or `is_author()`'s test will [fail immediately](https://github.com/GigaOM/legacy-pro/blob/master/application/wordpress/wp-includes/query.php#L3822).

So this leaves us with two options I think. The "author" taxonomy query has to be there for coauthors-plus to work. And we have to leave the `is_author` flag to `TRUE`. The only choice left is whether we should blank out the queried `author_name` and `author` as we've been doing [here](https://github.com/GigaOM/go-local-coauthors-plus/blob/a6269a6244a02b0f07cd0ac9976b2e5232b699ca/components/class-go-local-coauthors-plus-query.php#L119). I think we might be able to get away with it since `parse_query` is called after WP_Query has already processed for author queries. I think leaving `is_author` set to `TRUE` might give us the desired results but I'd have to test it first.