DefaultBackoff does not incorporate timeout which may lead to hanging requests

If a server sets the waiting time for the status "429 TooManyRequests" to 24 hours or even more, the client will hang for this amount of time. This scenario is not uncommon for servers that implement drastic rate limits.

This line in the [backoff function](https://github.com/hashicorp/go-retryablehttp/blob/02c1586c8f14be23e7eeb522f1094afbabf45e93/client.go#L474)
```
if resp.StatusCode == http.StatusTooManyRequests || resp.StatusCode == http.StatusServiceUnavailable
```
allows to wait if the server currently does not accept requests.

You can set a timeout for the http.client: `client.HTTPClient.Timeout = time.Duration(timeout) * time.Second` but it does not apply to the DefaultBackoff function.

Is this the desired default behavior?

---------------

This is the fix in my code I use. It currently still retries but at least it's not hanging:
```
func NewBackoff(min, max time.Duration, attemptNum int, resp *http.Response) time.Duration {
timeout := int64(30) // define timeout again
if resp != nil {
if resp.StatusCode == http.StatusTooManyRequests || resp.StatusCode == http.StatusServiceUnavailable {
if s, ok := resp.Header["Retry-After"]; ok {
if sleep, err := strconv.ParseInt(s[0], 10, 64); err == nil {
if sleep > timeout { // waiting time supplied by server must not exceed timeout
return time.Duration(0)
}
return time.Second * time.Duration(sleep)
}
}
}
}

mult := math.Pow(2, float64(attemptNum)) * float64(min)
sleep := time.Duration(mult)
if float64(sleep) != mult || sleep > max {
sleep = max
}
return sleep
}
```

Is there a better fix? I can offer a pull requests if wanted.