message: fix error when serialize and deserialize back  0
diff: @@ -148,7 +148,7 @@ fn to_hex(bytes: &[u8], skip_leading_zero: bool) -> String {
  let non_zero = bytes.iter().take_while(|b| **b == 0).count();
  let bytes = &bytes[non_zero..];
  if bytes.is_empty() {
  - return "0".into();
  + return "00".into();
  } else {
  bytes
  }
  @@ -186,8 +186,9 @@ mod tests {
  serde_json::to_string_pretty(&U256::from(1024)).unwrap()
  );
  assert_eq!(
  - "\"0\"",
  + "\"00\"",
  serde_json::to_string_pretty(&U256::from(0)).unwrap()
  );
  + assert_eq!(U256::from(0), serde_json::from_str("\"00\"").unwrap());
  }
  }
  