[FIX] Restore sign and encrypt status when a editing of a draft is continued [#760 state:fixed] [#692 state:fixed]

- Already decides whether or not to encrypt a draft in [ComposeBackEnd makeMessageWithContents:isDraft:shouldSign:shouldEncrypt:shouldSkipSignature:shouldBePlainText:]
- Saves the current sign and encrpyt status as messages headers, in order to restore them later.
Merge branch 'dev' of https://github.com/GPGTools/GPGMail into dev