Fix stale responses with no Last-Modified.

When caching a request with headers like this:

    Cache-Control: max-age=300, public
    ETag: "5c564d692c85b70068fd76ea25f2be2f"

Everything works fine while fresh, but once stale, an error is being raised from within `Net::HTTP`, since there's no `Last-Modified` header.

See #85 for the full background.

I spent around an hour trying to write a failing spec for this, but was unsuccessful. Maybe someone with more knowledge of this gem and its test environment would be more successful.