Register a route with by specifying default request arguments #617

Currently we have:

register_json_route( 'wp', '/users/me', array(
     'methods'         => 'GET',
     'callback'        => array( $controller, 'get_current_item' ),

I should be able to register a route to get_item() with some default argument to get the same effect without having to create a standalone get_current_item() method.

Another use case is an endpoint with the "top posts".