This has been wrong from the outside, but hidden by the fact that the
default values are equal. I've changed to MD5Hash which actually asserts
that the correct beatmap has likely arrived.
Found this in my realm changes, where it fails due to the beatmap ID
being a differing Guid in each case.
The main goal here is to remove the inheritance, since realm doesn't
like that. Unfortunate that we can't use object initialisers in a few of
these places, but no real way around that.
