metadata cache file rewritten every time extend_cache resource is fetched

[jmarantz]

See the first comment in #1085 for the observation from @stevemayhew

[stevemayhew]

Thanks @jmarantz! This is great. If you have a branch / fork I'm happy to fetch, build and test it in our environment. The issue does not seem to bother chrome, but other browsers may be more fragile in their expectations of cache headers.