code: func (c HTTPHeadersCarrier) ForeachKey(handler func(key, val string) error) error {
  	for k, vals := range c {
  		for _, v := range vals {
  			if err := handler(k, v); err != nil {
  				return err
  			}
  		}
  	}
  	return nil
  }
docs: // ForeachKey conforms to the TextMapReader interface.