message: fix Arbitrary of block header
diff: @@ -347,7 +347,7 @@ mod test {
  block_content_size: Arbitrary::arbitrary(g),
  block_content_hash: Arbitrary::arbitrary(g),
  block_parent_hash: Arbitrary::arbitrary(g),
  - chain_length: Arbitrary::arbitrary(g),
  + chain_length: ChainLength(Arbitrary::arbitrary(g)),
  }
  }
  }
  