Retry after context cancel

Hello I have a use case where I'd like to impose a timeout on my http client with retries. This means that http client's context cancellation should be retryable action. According to my testing it is not:

```go
func TestRetryableHTTP(t *testing.T) {
client := retryablehttp.NewClient()
client.CheckRetry = func(ctx context.Context, resp *http.Response, err error) (bool, error) {
if ctx.Err() != nil {
return true, ctx.Err()
}
return retryablehttp.DefaultRetryPolicy(ctx, resp, err)
}

attempts := 0
handlerWithDelay := func(delay time.Duration) http.HandlerFunc {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
attempts++
ticker := time.NewTicker(delay)
select {
case <-r.Context().Done():
t.Log("connection droppped from client")
return
case <-ticker.C:
}
t.Log("responding 200 OK")
w.WriteHeader(http.StatusOK)
})
}
ts := httptest.NewServer(handlerWithDelay(10 * time.Second))
defer ts.Close()

ctx, cancelFn := context.WithTimeout(context.Background(), time.Second)
defer cancelFn()
req, err := http.NewRequestWithContext(ctx, http.MethodGet, ts.URL, nil)
require.NoError(t, err)

retReq, err := retryablehttp.FromRequest(req)
require.NoError(t, err)
_, err = client.Do(retReq)
assert.NoError(t, err)

assert.Equal(t, 5, attempts) // 4 retries + first call

}
```
Test output:
```
=== RUN TestRetryableHTTP
2022/05/16 19:18:41 [DEBUG] GET http://127.0.0.1:49948
2022/05/16 19:18:42 [ERR] GET http://127.0.0.1:49948 request failed: Get "http://127.0.0.1:49948": context deadline exceeded
2022/05/16 19:18:42 [DEBUG] GET http://127.0.0.1:49948: retrying in 1s (4 left)
sanehttp_test.go:168: connection droppped from client
sanehttp_test.go:187:
Error Trace: sanehttp_test.go:187
Error: Received unexpected error:
context deadline exceeded
Test: TestRetryableHTTP
sanehttp_test.go:189:
Error Trace: sanehttp_test.go:189
Error: Not equal:
expected: 5
actual : 1
Test: TestRetryableHTTP
--- FAIL: TestRetryableHTTP (1.01s)
```

You can see there was only 1 attempt and no retries.

I have also tried to use the http client `Timeout` field instead of context cancellation but it yields the same result.

I was checking the source code and it looks like you are not retrying on errors from `client.Do()` which is the case. Do you know if there is a way how to make my use case work?

Thank you.