Fill public fields of jose.Signature after signing

[apoloval]

When jose.Signer receives a Sign() call, it produces a *JSONWebSignature object that only have the raw/private fields of the jose.Signature filled. The public fields such as Header, Protected and Unprotected have zero values. As they are only used when JSONWebSignature is parsed. This causes an inconsistency and misleading behavior, as one would expect that a signature created by the signer reflects the right values in the headers.