code: func (kr *GroupResolver) SelectKeys(req *http.Request) []Key {
  	keyName := req.Header.Get(kr.keyHeader)
  	kr.RLock()
  	defer kr.RUnlock()
  	if keyName != "" {
  		return kr.keyMap[keyName]
  	}
  	var keys []Key
  	for _, ks := range kr.keyMap {
  		keys = append(keys, ks...)
  	}
  	return keys
  }
docs: // SelectKeys returns the keys in the group with the name identified by the request key selection
  // header. If the header does value does not match a specific group then all keys are returned.