code: func (c *Client) GetJSON(ctx context.Context, url string, data interface{}) error {
  	if !strings.HasPrefix(url, c.discoRoot()) {
  		return fmt.Errorf("wrong URL (%q) for this server", url)
  	}
  	hreq := c.newRequest(ctx, "GET", url)
  	resp, err := c.expect2XX(hreq)
  	if err != nil {
  		return err
  	}
  	return httputil.DecodeJSON(resp, data)
  }
docs: // GetJSON sends a GET request to url, and unmarshals the returned
  // JSON response into data. The URL's host must match the client's
  // configured server.