Ah, good catch about the header comment!
If this is being fixed upstream, I suppose it does not make sense to
patch it in go-message. Though, it doesn't look like that CL has been
applied yet and since go 1.13 is frozen we won't see it until at least
February 2020 with 1.14. In the meantime, I have a few broken messages
in aerc, so I'm not quite sure what to do about that. Doesn't seem like
this is affecting too many people based on the activity on that github
issue, so maybe I'll just keep it patched locally until go is fixed.
I'd be interested by a patch in go-message. I think there are cases
(apart from Date and addresses) in which we should ignore CFWS anyway.
Seems to me like this is a header field comment (so there could be
anything inside the parentheses).
See  for the definition of CFWS and  for the definition of
date-time inside Date header fields:
date-time = [ day-of-week "," ] date time [CFWS]
Note the trailing CFWS.
It seems like there is already a Go patch for this .