I noticed that my `AuthenticationError`'s `status_code` field was 401, but when I ran `.as_response_error()` the `ResponseError`'s code was 500 (the default value). It's because impl ResponseError for AuthenticationError doesn't define the `status_code()` trait method. Merely setting the status code in `error_response` isn't enough, it seems. I added a unit test for this case too.