Up to this point, we've tested protected headers on messages that have
either been encrypted or signed, but not both.
This adds a couple tests of signed+encrypted messages, one where the
subject line is masked (outside subject line is "Subject Unavailable")
and another where it is not (outside Subject: matches inner Subject:)
See the discussion at
https://dkg.fifthhorseman.net/blog/e-mail-cryptography.html#protected-headers
for more details about the nuances between signed, stripped, and
stubbed headers.
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
