Fix verifying encapsulated messages

This monkeypatches mail-gpg to restore the use of the unaltered raw source of the relevant email part. This code was removed in https://github.com/jkraemer/mail-gpg/commit/5fded41ccee4a58f848a2f8e7bd53d11236f8984 for yet unknown reasons.

This also adds a test case to prevent future regressions.

And moves one older monkeypatch into the right directory.

Closes #502 (closed)