code: func (c Client) WithHeader(k, v string) Client {
  	if v == "" {
  		return c
  	}
  	if c.headers == nil {
  		c.headers = map[string]string{}
  	}
  	c.headers[k] = v
  	return c
  }
docs: // WithHeader returns a shallow copy of the client with a header to include the
  // requests.