remove cache-control header

[gka]

This PR resolves the outstanding maxage header issue. A fixed maxage header is not practical. There should be either a way to customize the maxage, or, more simple, the Sapper middleware shouldn't even set it at all.

The solution is to remove the res.setHeader('Cache-Control', dev ? 'no-cache' : 'max-age=600'); line and the corresponding test.

[benmccann]

The current behavior seems extremely severely broken to me. This code path only serves dynamic content, so the expectation would certainly be that nothing is cached.

There's a number of users reporting issues in #567 with well thought out and described issues. This includes Sapper regulars such as @antony who described this as "mission critical" and "very high priority" because the current behavior results in "weird half-broken state for all my users" #567 (comment)

[benmccann]

@antony is this a breaking change? It changes behavior, but I'm having a hard time envisioning a scenario where it's not backwards compatible or breaks something

[antony]

@benmccann if you remove the cache-control header (which a number of CDNs use to decide how to cache your pages) it will certainly change the way your site operates, which I'd suggest is breaking.