Account rollover inner JWS should not include a nonce

[miken32]

Per section 7.3.5, "the inner JWS MUST omit the "nonce" header parameter."

I'm not fluent in Go, but from what I understand of the signing code you could set a flag that allows nil to be passed.

I'm aware this is largely unmaintained code; I'm posting this issue more as a caveat to anyone else that might use it than as a request for it to be fixed.