Call resolve-key in sign function on pkey

[janetacarr]

Hey Buddy!

I'm not sure if this entirely qualifies as an issue, so close it if not, but I just discovered that you can sign tokens with nil as the pkey/secret, but not unsign the same token with nil as there's no implementation of IKeyProvider (used by unsign ala resolve-key here )

So, should there be a call to resolve-key in the sign function? found here

Anyways, let me know if this was intended or not. If not, I'll open a PR.