RFE: hardcode fields required by RFC in anonymized posts
See this thread: https://lists.mailman3.org/archives/list/mailman-users@mailman3.org/message/YEJJRRLQ236RTW2BYXTKRRV6BHYO5FCH/. Summary: if anonymous_list_keep_headers doesn't contain the regexp "^(?!x-)" headers essential to Mailman processing such as Content-Type and Message-ID will be deleted.