updated Mail::GpgExtension to work with Ruby Mail in Stretch
[fix] allow any disposition for GPG mail parts
RFC3156 does not specify anything about Content-Disposition.
followup on c3722b8
[fix] the signature can be an attachment or not (from ruby-mail's point of view), it does not matter (depends on the MUA's preferences)
[fix] trust keys used to encrypt
[fix] fixed several silly mistakes when mail/gpg was created
[evol] ported the GPG extension from TMail to Mail (CyborgHood::Mail needs to be adapted now)