HTTP response fixes (increase cacheability)

[da2x]

The 301 status means clients should never use the redirector to ask for a new mirror again, and direct all future requests to the one mirror they were redirected to on the first request. This is not what is intended here. A 307 temporary redirect is the correct response. Fixes #65.

Let clients and proxies cache responses from the redirector for 7:30 minutes instead of refetching the redirect from the redirector on every request within a short time frame. Makes the redirector play nicer with caching proxies who can handle mass-updates on a local network locally without refetching the remote resource for every request. Fixes #66.

See also my blog post.

Finally, add a Server header as it’s one of the few headers required by HTTP. Gave it the vanity name “http-redirector”.