Horses do not convert their owner to UUID

Wolves and ocelots have code that will read an owner name, if one exists, and convert it to an owner UUID. Horses, on the other hand, simply drop their code for owner names and now only work with UUIDs. This means all horses lose their owner when upgrading. Vanilla does not use this information but it does store it and mods have come to rely on it. The /testfor command allows matching an entity with a given NBT tag so command blocks could be relying on it as well.