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.

Resolves #85.