allow category agnosticity in user resource's update method

The current implementation of the update method of the user resource requires a separate API call to update each category. This is surprising for users who use the core Profile module to add fields to profiles under several categories. The attached patch modifies the user resource's update method so that if no category is specified, all categories will be updated automatically without the need to update categories one-by-one.

If this patch isn't acceptable, the alternative is to use it as a basis for an override of the user update method via hook_services_resources_alter.