Mail::AuthenticationResults provides object-oriented authentication-results
email headers. This parser copes with most styles of Authentication-Results
header seen in the wild, but is not yet fully RFC7601 compliant.

Differences from RFC7601:
- key/value pairs are parsed when present in the authserv-id section, this is
  against RFC but has been seen in headers added by Yahoo!.
- Comments added between key/value pairs will be added after them in the data
  structures and when stringified.