implement s-maxage support #47

[dpwspoon]

This is an ok first implementation. But have you thought about s-maxage and maxage in terms of a shared cache for different Authorization Headers?

Example:

Cached asset for request with Authorization: ABC

request from application with Authorization: XYZ

if cache has valid s-maxage then we should be able to return (I would think)

if cache has valid max-age but not s-maxage, we should not return it from cache.

[hthetiot]

@dpwspoon for Authorization header see:

• https://github.com/kaazing/http2-cache.js/pull/58