Fixed a bug in MediaObject.serialize_resource_list() when called more than once on the same object

MediaObject.serialize_resource_list() was supplying an HTTP server
 MediaResource uri prior to serialization when it was blank but didn't
 clear it back. So the first uri was always retained. Restructured
 the code to not touch the MediaResource as part of the serialization
 process.

Went ahead and renamed MediaResource.write_didl_lite() to serialize().
 This is consistent with other librygel-server objects.