add_coauthors() deleting old users if user_login is email address

I noticed this issue if the `user_login` is different from `user_nicename`. This occurs in the main file `co-authors-plus.php` on line `815`:

`if ( $append ) {
$existing_coauthors = wp_list_pluck( get_coauthors( $post_id ), 'user_login' );
} else {
$existing_coauthors = array();
}`
The problem is that my `user_login` is the same as `user_email` and `user_nicename` removes the `@` and `.` from the email address making it very different from the `user_login`. Also, if anywhere in the plugin it checks for `user_nicename` shouldn't this check for it too?

Start in co-authors-plus.php around line 815 and trace how add_coauthors() compares existing authors with the supplied login values. Reproduce the case where user_login differs from user_nicename, then verify that existing coauthors are preserved and the requested authors are handled correctly.

Written by the indexing model from the issue text.