code: func HeaderExtractor(headers []string) func(*http.Request) string {
  	return func(r *http.Request) string {
  		for _, h := range headers {
  			v := r.Header.Get(h)
  			if v != "" {
  				return v
  			}
  		}
  
  		return ""
  	}
  }
docs: // HeaderExtractor returns a function that can extract a value from a list
  // of headers.