Tell the client the connection is closing after each response

[arpitjain099]

Fixes #211.

handleHTTPConnection and handleTLSConnection each read one request and close the connection in a defer, but neither response says so. The upstream Connection: keep-alive is not passed through either, so a client that pools sockets, Node's https.Agent({keepAlive: true}) in the report, keeps the socket and the next write lands on a connection the proxy has already gone away from. That is the alternating socket hang up in the issue, and it is why code-server --install-extension fails outright and npm install crawls.

RFC 9112 section 9.6 puts that on the sender: if you are going to close, say so. The forwarded response now drops any upstream Connection header and sets resp.Close, so Response.Write emits Connection: close, and the blocked 403 does the same.

This is the first of the two options in the issue. Actually keeping connections alive, the second option, is the better outcome for throughput, but it means holding the connection open and looping over requests in both handlers, which is a larger change and a design call for you rather than something to slip into a bug fix.

The test starts a local backend, goes through the proxy for one allowed and one blocked request, and asserts resp.Close on both. It fails on main with "response must tell the client the connection is closing". go test -race ./proxy/ passes.