dwhois.whois: fix encoding issue by coercing whois to bytes

bson expects strings to be valid utf-8 but the output of whois isn't
necessarily in any particular encoding, or at least it won't tell you
what encoding it was in if it did know.

This caused an exception to be raised:

bson.errors.InvalidStringData: strings in documents must be valid UTF-8

So we coerce the output of whois to bytes, since that's all we can
guarantee that it is and let bson encode it how it wants to.