message: add unit test to show we fail to decrypt a HDPayload that is too large
diff: @@ -232,6 +232,21 @@ mod tests {
  assert_eq!(bytes, key.decrypt(&payload).unwrap())
  }
  + #[test]
  + fn decrypt_too_large() {
  + const TOO_LARGE_PAYLOAD : usize = 2 * MAX_PAYLOAD_SIZE;
  + let bytes = vec![42u8; TOO_LARGE_PAYLOAD];
  + let seed = hdwallet::Seed::from_bytes([0;hdwallet::SEED_SIZE]);
  + let sk = hdwallet::XPrv::generate_from_seed(&seed);
  + let pk = sk.public();
  +
  + let key = HDKey::new(&pk);
  + match key.decrypt(&bytes).unwrap_err() {
  + Error::PayloadIsTooLarge(len) => assert_eq!(len, TOO_LARGE_PAYLOAD - TAG_LEN),
  + err => assert!(false, "expecting Error::PayloadIsTooLarge({}) but got {:#?}", TOO_LARGE_PAYLOAD - TAG_LEN, err),
  + }
  + }
  +
  #[test]
  fn path_cbor_encoding() {
  let path = Path::new(vec![0,1,2]);
  