Require a public key in DummyRedeemer

DummyRedeemer allows None as a value for its public key. It is never legitimate for a real issuer to not offer us a public key. Doing so completely breaks the security model for the client (they lose the ability to prove the issuer is not tracking them by using a custom signing key).

So, make DummyRedeemer more like real usage and have it always have a public key.

This is motivated by #110 (closed) where the client is going to start looking at the public key more carefully and at best it would be a lot of unnecessary complexity to handle None public key from DummyRedeemer.