Default roles not granted when joining moderated groups

Issue Summary

Subscription requests must be approved when groups are moderated. When the request is approved, the user joins the moderated group but is not granted the roles checked in the OG Default Roles module admin page.

The reason is the following:
The implementation of hook_og in og_default_roles.module only check for 'user insert' and 'user delete' operations. When a request is approved, og_default_roles_og() is called with $op set to 'user update', resulting in no action.