Remove duplicate orderby for users query

[rebeccahum]

Description of the Change

We noticed that there was a typo for the users query resulting in a duplicate orderby clause:

ElasticPress/includes/classes/Indexable/User/User.php

Line 735 in b14ab9d

$objects = $wpdb->get_results( $wpdb->prepare( "SELECT SQL_CALC_FOUND_ROWS ID FROM {$wpdb->users} ORDER BY %s %s LIMIT %d, %d", $args['orderby'], $args['orderby'], (int) $args['offset'], (int) $args['number'] ) );

This resulted in the below query:

SELECT SQL_CALC_FOUND_ROWS ID FROM wp_users ORDER BY ‘ID’ ‘ID’ LIMIT 0, 350

sanitize_sql_orderby() is for sanitizing the ORDER BY clause for variable use in the $wpdb statement. Variable usage is for avoiding quotes since $wpdb->prepare() uses quotes in placeholders and that doesn't play nicely with mariadb.

Props @brettshumaker Automattic#97, @pschoffer https://github.com/Automattic/ElasticPress/pull/112/files

Alternate Designs

N/A.

Benefits

No more duplicate orderby clause!

Possible Drawbacks

None that I can tell.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Fixed: Duplicate orderby statement in Users query.