message: Let assert_conversion take StdError directly
diff: @@ -251,8 +251,7 @@ mod test {
  assert_ne!(error1, error2);
  }
  - fn assert_conversion(r: StdResult<()>) {
  - let original = r.unwrap_err();
  + fn assert_conversion(original: StdError) {
  let seralized = to_vec(&original).unwrap();
  let restored: StdError = from_slice(&seralized).unwrap();
  assert_eq!(restored, original);
  @@ -260,7 +259,7 @@ mod test {
  #[test]
  fn generic_err_conversion() {
  - assert_conversion(GenericErr { msg: "something" }.fail());
  + assert_conversion(GenericErr { msg: "something" }.build());
  }
  #[test]
  @@ -269,28 +268,28 @@ mod test {
  InvalidBase64 {
  msg: "invalid length".to_string(),
  }
  - .fail(),
  + .build(),
  );
  }
  #[test]
  fn unauthorized_conversion() {
  - assert_conversion(Unauthorized {}.fail());
  + assert_conversion(Unauthorized {}.build());
  }
  #[test]
  fn null_pointer_conversion() {
  - assert_conversion(NullPointer {}.fail());
  + assert_conversion(NullPointer {}.build());
  }
  #[test]
  fn not_found_conversion() {
  - assert_conversion(NotFound { kind: "State" }.fail());
  + assert_conversion(NotFound { kind: "State" }.build());
  }
  #[test]
  fn parse_err_conversion() {
  - let err = from_slice::<String>(b"123").map(|_| ());
  + let err = from_slice::<String>(b"123").unwrap_err();
  assert_conversion(err);
  }
  @@ -301,7 +300,7 @@ mod test {
  source: "Person",
  msg: "buffer is full",
  }
  - .fail(),
  + .build(),
  );
  }
  }
  