handler: even for valid pgp encrypted emails, decode octetstream

In some cases, the octet stream may be encoded, even though the multipart looks like a PGP valid encryped email.

In particular, I've noticed this because of the new bugzilla.redhat.com Secure Email feature, which sends encrypted emails. However, they have the octet-stream encoded with quoted-printable, which makes current version of mutt fail when opening the email. This patch always runs the octetstream through the decoder, which chooses whether to decode the octetstream or not.