[FIX] Force Mail.app to fetch PGP signed emails just like S/MIME signed messages so the signature isn't corrupted [#600 state:fixed]

- Mail.app doesn't use the BODYSTRUCTURE IMAP information now, to build the
  mime parts of the message, but as with S/MIME it parses the entire body.
- Building the mime parts from BODYSTRUCTURE corrupted the signature, since
  the returned information might be using different casing than the original message.
Merge branch 'dev' of github.com:GPGTools/GPGMail into dev