Be explicit with encodings in the gateway.

The latest python3 moksha/fedmsg stuff makes it such that things are
more explicitly bytes/unicode on the frontiers/core of fedmsg.  Here, we
have to adopt that same order in the gateway too (which is kind of an
edge service anyways).
Merge pull request #249 from fedora-infra/feature/explicit-encoding

Be explicit with encodings in the gateway.