verify_signed_message was not checking for the validity of the
signature. The documentation of python-gnupg is not clear about this, it
says that the fiels are set just if the signature is valid.
In this case tests/robots/1/signed_message was signed with the correct
key but with the wrong digest-algo (SHA256 instead of SHA512) as
expressed on the signature.
Running gpg --verify tests/robots/1/signed_message returned:
gpg: WARNING: signature digest conflict in message
gpg: Can't check signature: General error
and a non zero error code, but verify_signed_message was not catching
this because verified.fingerprint was set even though verified.valid was
False.
