code: func (c *cache) Header(key string) (Header, error) {
  	path := headerPrefix + formatPrefix + hashKey(key)
  	f, err := c.fs.Open(path)
  	if err != nil {
  		if vfs.IsNotExist(err) {
  			return Header{}, ErrNotFoundInCache
  		}
  		return Header{}, err
  	}
  
  	return readHeaders(bufio.NewReader(f))
  }
docs: // Retrieve the Status and Headers for a given key path