public AcmeClient(Uri rootUrl = null, AcmeServerDirectory dir = null,
ISigner signer = null, AcmeRegistration reg = null)
{
RootUrl = rootUrl;
Directory = dir;
Signer = signer;
Registration = reg;
UserAgent = string.Format(AcmeProtocol.HTTP_USER_AGENT_FMT,
this.GetType().Assembly.GetName().Version);
}