http.go
func (c *Client) GetResource(ctx context.Context, pathPart string, sid string, v interface{}) error {
	sidPart := strings.Join([]string{pathPart, sid}, "/")
	return c.MakeRequest(ctx, "GET", sidPart, nil, v)
}
func (c *Client) GetResource(ctx context.Context, pathPart string, sid string, v interface{}) error {
	sidPart := strings.Join([]string{pathPart, sid}, "/")
	return c.MakeRequest(ctx, "GET", sidPart, nil, v)
}
// GetResource retrieves an instance resource with the given path part (e.g.
// "/Messages") and sid (e.g. "MM123").