Multipart menu_path's are not working

Problem

Modules can set or alter a resource's endpoint through hook_restws_resource_info[_alter]() via the menu_path item. One of the examples given in restws.api.php is setting the menu path for 'Book' to 'api/mybook'. We are trying to use the menu_path property to change the resource endpoints on drupal.org (exposing for example the node resource under /api-d7/node/12345.json instead of /node/12345.json), see #1710850: Deploy RestWS for D7 project issue JSON.

However, restws_page_callback is hardcoded to always use the first element as resource, the second element as resource id. Hence, any menu_path containing more than one part will result in a 404.

/**
 * Menu page callback.
 */
function restws_page_callback($resource, $page_callback = NULL) {
  $id_arg = arg(1);
  $resource_arg = arg(0);
  $format = FALSE;
  $id = NULL;

Proposed solution

If set, take the menu_path property of a resource into account when determining the position of resource and resource ID in the path.