message: fix get_header_header
diff: @@ -32,7 +32,11 @@ impl Network {
  // TODO return BlockHeader not MainBlockHeader
  fn network_get_head_header(storage: &storage::Storage, net: &mut Network) -> blockchain::BlockHeader {
  - let mbh = GetBlockHeader::first().execute(&mut net.0).expect("to get one header at least");
  + let block_headers = GetBlockHeader::tip().execute(&mut net.0).expect("to get one header at least");
  + if block_headers.len() != 1 {
  + panic!("get head header return more than 1 header")
  + }
  + let mbh = block_headers[0].clone();
  tag::write(&storage, &HEAD.to_string(), mbh.get_previous_header().as_ref());
  mbh
  }
  