Get Items: Retrieve the data using WordPress coding standards

[kjbenk]

When I was creating some unit tests for the /users endpoint I noticed that the get_items() function was retrieving meta data using a custom $wpdb->get_results() query.

I think this is only a temporary patch so this issue can be the formal way for us to make sure that it is implemented correctly before release :)

Reference - https://github.com/WP-API/wp-api-meta-endpoints/blob/master/lib/class-wp-rest-meta-controller.php#L170