Mail::DKIM::Signer wraps DKIM-Signature with with \r\n\t; this
is the hardcoded Separator passed to Mail::DKIM::TextWrap via
Mail::DKIM::KeyValueList. MIME::Tools on the other hand
(MIME::Head::stringify() in particular) encode EOL as plain \n;
it is thus necessary to normalize CRLF->LF for DKIM-Signature to
avoid confusing the mail agent.