no-cache + no-store = no etag

Regarding the NoCache filter. It sets both no-cache and no-store. But I was looking for a solution that only sets no-cache. So I can still benefit from a revalidation using Etags. See e.g. this article on developers.google which explains the principles excellently.

So I propose making no-store optional. I'll make a pull request.

By the way, I think no-store obsoletes no-cache, since it would not allow storing anything at all to begin with. But I suppose it does no harm to have both.