message: Fix arbitrary instance for Message
diff: @@ -172,7 +172,7 @@ mod test {
  2 => Message::StakeDelegation(Arbitrary::arbitrary(g)),
  3 => Message::StakePoolRegistration(Arbitrary::arbitrary(g)),
  4 => Message::StakePoolRetirement(Arbitrary::arbitrary(g)),
  - 4 => Message::OldUtxoDeclaration(Arbitrary::arbitrary(g)),
  + 5 => Message::OldUtxoDeclaration(Arbitrary::arbitrary(g)),
  _ => Message::Transaction(Arbitrary::arbitrary(g)),
  }
  }
  