Fix cache

[mhluska]

In my case the cache wasn't working because res.send was never being called (thus cache.put was never called).

After changing the implementation to use res.write and res.end and getting it to work, I noticed that the cache can only be used for caching text/html resources because the res.send() call for a cache hit will overwrite the content type, regardless of what type it is storing (css, js etc). So you have to be careful to have a cache implementation that rejects a promise during the fetch call if the content type is any besides text/html.

Also, the cache middleware has to go before gzip otherwise gzip doesn't work.