client.go
func HTTPClient(h *http.Client) ClientParam {
	return func(c *Client) error {
		c.client = h
		return nil
	}
}
func HTTPClient(h *http.Client) ClientParam {
	return func(c *Client) error {
		c.client = h
		return nil
	}
}
// HTTPClient is a functional parameter that sets the internal
// HTTPClient of the roundtrip client wrapper