A downside of using gob is that the cache file is no longer readable/editable directly.
I liked that feature as part of httpcache's simplicity as it allows easy debugging of http libraries, e.g. https://github.com/StalkR/imdb.

Since all we want is to save the date, I was thinking we could just use the file system date of the file:
```
func touch(filename string) error {
  now := time.Now()
  if err := os.Chtimes(filename, now, now); err != nil {
    return err
  }
  return nil
}

func (f fileCache) Put(u *url.URL, data []byte) error {
  filename := f.fileName(u)
  if err := ioutil.WriteFile(filename, data, 0644); err != nil {
    return err
  }
  if err := touch(filename); err != nil {
    return err
  }
  return nil
}
```

And in Get(), do the reverse by reading entry.SaveTime from modification time of the file.