box: fix transfers getting stuck on token expiry after API change

Box recently changed their API, changing the case of returned API items

> On May 10th, 2021, as part of our continued infrastructure upgrade,
> Box's API response headers will standardize to return in a case
> insensitive manner, in line with industry best practices and our API
> documentation. Applications that are using these headers, such as
> "location" and "retry-after", will need to verify that their
> applications are checking for these headers in a case-insensitive
> fashion.

Rclone was reading the raw headers from the `http.Header` and not
using the `Get` accessor method which meant that it was sensitive to
case changes.

This fixes the problem by using the `Get` accessor method.

See: https://forum.rclone.org/t/box-backend-incompatible-with-box-api-changes-being-deployed/22972

backend/box/box.go

@@ -323,7 +323,7 @@ func shouldRetry(ctx context.Context, resp *http.Response, err error) (bool, err
 	}
 	authRetry := false
 
-	if resp != nil && resp.StatusCode == 401 && len(resp.Header["Www-Authenticate"]) == 1 && strings.Index(resp.Header["Www-Authenticate"][0], "expired_token") >= 0 {
+	if resp != nil && resp.StatusCode == 401 && strings.Contains(resp.Header.Get("Www-Authenticate"), "expired_token") {
 		authRetry = true
 		fs.Debugf(nil, "Should retry: %v", err)
 	}