Fix bugs with auth serializer

- The auth serializer improperly handles packing/unpacking of signed data, in that signatures containing \x00\x01 will cause the unpacking operation to fail (and
celery to crash.
  - The included fix is very naive. It simply double encodes/decodes the signature with b64. A more proper solution would be to use length prefixed strings (or
fixed length reads since the sizes are easily calculatable), but I'm lazy.

- Additionally, using auth with events was difficult because setup_security removes all serializers, but does not set a default serializer. Added a call to
 _set_default_serializer in register_auth.
Typo decode->encode