Double proposal proof signature verification generates the wrong data to be signed

[hrxi]

This means that it's essentially impossible to generate double proposal proofs currently.

Currently, the double proposal proofs simply sign the Blake2b hash of the macro header, but the correct data can be seen in the following:

core-rs-albatross/validator/src/tendermint.rs

Lines 364 to 378 in 76cc30b

	fn sign_proposal(
	&self,
	proposal_message: &ProposalMessage<Self::Proposal>,
	) -> Self::ProposalSignature {
	let data = SignedProposal::hash(
	&proposal_message.proposal.0,
	proposal_message.round,
	proposal_message.valid_round,
	)
	.serialize_to_vec();
	(
	self.block_producer.signing_key.sign(&data),
	self.validator_slot_band,
	)
	}