column_posts() should use user_login when querying for linked_account

[TheCrowned]

This fixes #554.

In the Guest Authors view, some authors would have a wrong post count. This would happen for users with a user_login different than the user_nicename.

Up to now, due to this Core issue, count_user_posts() would return the total site posts count for those users. When that bug will be fixed, it will return 0. In any case, it is not the correct count.

The issue lies in this part of the code:

if ( ! empty( $item->linked_account ) && $guest_term->count ) {
	$count = count_user_posts( get_user_by( 'slug', $item->linked_account )->ID );

because $item->linked_account contains the user login rather than the slug (which would be the nicename). Besides returning a wrong count, it would also generate a bunch of PHP Notices because get_user_by would return false and we would thus be fetching property of non-object.

[mdbitz]

This looks good to me I'm seeing counts returned based on the login and no notices in my test install.

[mdbitz]

Approved, note unit test is pre-existing failure.