Status 401 Authentication failures do not result in retry behavior

I'm trying out retryablehttp, and seeing behavior that I'm not sure is a bug or just my expectation being faulty.

I have an http call to an external service that occasionally, inexplicably, fails with a 401 Authentication error.

Calls to that service in my sample code look like this:

```
req, _ := http.NewRequest("GET", reqURL, nil)
req.Header.Add("Authorization", authHeader)
retryClient := retryablehttp.NewClient()
retryClient.RetryMax = 3

retryreq, err := retryablehttp.FromRequest(req)
if err != nil {
...
}
resp, err := retryClient.Do(retryreq)
```

Here's what I observe:

1. If I force the value for `reqURL` to be an invalid domain, then I see the retry behavior I'd expect in the logs... it tries several times, with exponential backoff, and it's clearly visible in the logs
2. If I use the correct URL, but instead force an authentication failure by passing in incorrect auth credentials, I *do not see* retry behavior... the calls immediately fail with a 401 error. I do see the initial debug log from retryablehttp, but that's it

Thanks for any clarity!