Registration reply should include same Link headers as new registration

[r0ro]

If we want to retrieve Registration info from the client key pair, we can send a new-reg request.
We then get a 409 error with 'Location' header set to the Registration resource url.
We then use an empty POST update request to this url.
In the reply we don't have the "next" nor "terms-of-service" Link headers

[jsha]

Currently the spec (https://github.com/letsencrypt/acme-spec/blob/master/draft-barnes-acme.md) only specifies those Link headers for new-registration responses, not for existing registrations. That seems like a bug in the spec: Can you send a pull request against the spec indicating that existing registrations should provide those link headers also? We need to merge the spec fix before we can merge the Boulder fix. Thanks for spotting this!

[r0ro]

done: letsencrypt/acme-spec#198