Cannot read the response body inside the `Backoff` function (I think)

From my testing, if you try to read the response body in the `Backoff` function, you get an error:
```
http2: response body closed
```

Is there any way to access the response body within that function? (In our case, the 'retry after' information is in the body of the response)

The code I'm testing is:
```
func newClient() http.Client {
retryClient := retryablehttp.NewClient()
retryClient.RetryMax = 3
retryClient.Backoff = LisaTest
return retryClient
}

func TestBackoff(min, max time.Duration, attemptNum int, resp *http.Response) time.Duration {
if resp != nil {
var target map[string]interface{}
err := json.NewDecoder(resp.Body).Decode(&target)
spew.Dump(err)
spew.Dump(target)
}
return 1 * time.Second
}
```

I could well be being thick here - would love to know if we're missing something simple (or it's some part of our implementation that I've missed)!