Fix bugs around external body files

Possibly some naivete here. I'm giving body files unique-ish names using an MD5 of the HTTP verb and URI of the request. This means passing the request object around inside MemoryTape. Wonder if making responses aware of the requests they're responding to is a cleaner solution?

The upshot of this is, betamax-core tests pass, allowing the betamax-proxy build to fail horribly but visibly.