code: func (mem *Memory) IsExist(key string) bool {
  	if ret, ok := mem.data[key]; ok {
  		if ret.Expired.Before(time.Now()) {
  			mem.deleteKey(key)
  			return false
  		}
  		return true
  	}
  	return false
  }
docs: // IsExist check value exists in memcache.