// Sign given request using Signature V4.
func signStreamingRequest(req *http.Request, accessKey, secretKey string, currTime time.Time) (string, error) {
	// Get hashed payload.
	hashedPayload := req.Header.Get("x-amz-content-sha256")
	if hashedPayload == "" {
		return "", fmt.Errorf("Invalid hashed payload")
	}

	// Set x-amz-date.
	req.Header.Set("x-amz-date", currTime.Format(iso8601Format))

	// Get header map.