Fix copy plugins

the low level method `CMSPlugin.copy_plugin` messes up the reference to the old plugin changing it to the new one
This has no consequence if the old plugin instance is not "reused", but it may affect lower level use (like in blueprint)
`plugin_instance` is a derived class, so `pk` points to `cmsplugin_ptr_id` (the derived class primary key) while `id` is the primary key of the parent class