code: func (k Key) Vary(varyHeader string, r *http.Request) Key {
  	k2 := k
  
  	for _, header := range strings.Split(varyHeader, ", ") {
  		k2.vary = append(k2.vary, header+"="+r.Header.Get(header))
  	}
  
  	return k2
  }
docs: // Vary returns a Key that is varied on particular headers in a http.Request