Make WP_User_Query::prepare_query() public

[scribu]

WP_User_Query tries to follow the pattern from WP_Query, where if you don't pass any args, it won't run the query.

The problem is that you can't do anything with that instance afterwards; both prepare_query() and query() accept no parameters.

With the patch, it's now possible to do this:

// construct $args somehow

$uq = new WP_User_Query;

$uq->prepare_query( $args );

var_dump( $uq->query_where );