Bug in coauthors_plus->search_authors()

I think I've pinpointed a bug in co-authors search_authors. We were having issues assigning some posts to some authors when creating a post.

Look at the following block of code:

``` php
foreach( $found_terms as $found_term ) {
$found_user = $this->get_coauthor_by( 'user_nicename', $found_term->slug );
if ( !empty( $found_user ) )
$found_users[$found_user->user_login] = $found_user;
}
```

get_coauthor_by() function uses wordpress function get_user_by to get user data. However it's passing the term's slug as second argument instead of user's nicename.

Start in the implementation of coauthors_plus->search_authors() and inspect the shown get_coauthor_by() call, then trace how the term slug and user nicename are represented. Reproduce the author-assignment case described in the issue; done means affected authors can be assigned when creating a post.

Written by the indexing model from the issue text.