Provide query result data to custom user columns

[JohnC28]

When the filter manage_users_custom_column is triggered, it sends an empty string for the value field.

Since it's easy enough to extend the WP_User_Query with extra fields, it would be good to have those fields passed through if they exist to save having to requery the database with the user_id parameter to get the data.

I have attached a patch to show what I mean.

I guess plugins might expect an empty string. An example:
​http://plugins.trac.wordpress.org/browser/document-repository/tags/0.2.4/user-roles.php#L82

Another option might be to pass $user_object as the fourth parameter.

The filter is also used in class-wp-ms-users-list-table.php:
http://core.trac.wordpress.org/browser/tags/3.5.1/wp-admin/includes/class-wp-ms-users-list-table.php#L262