[exim] ARC: no t= field added

Hello!

I'm playing now with new experimental ARC module and found that signing
option "timestamps" doesn't applies and as result t= field in ARC
signatures is absent.
Seems about the same tells Google:

Authentication-Results: mx.google.com;
        dkim=pass header.i=@yandex.ru header.s=mail header.b=AAU0yuLQ;
        arc=fail (missing mandatory fields);
...

--
With best regards,
Max Kostikov

T: +7(952)7927000 | W: https://kostikov.co | BBM: 24CA5DF8