module Net::IMAP::SASL::GS2Header

def gs2_authzid

If +#authzid+ is empty or +nil+, an empty string is returned.

+gs2-authzid+ header, when +#authzid+ is not empty.
The {RFC5801 ยง4}[https://www.rfc-editor.org/rfc/rfc5801#section-4] 
def gs2_authzid
  return "" if authzid.nil? || authzid == ""
  "a=#{gs2_saslname_encode(authzid)}"
end