Fix persistent (keep-alive) connections

[Frixuu]

Weblink currently closes persistent ("keep-alive") connections when the client tries to reuse them, which is kind of against the point of persistent connections.

As an example, the following application:

function main() {
    final app = new weblink.Weblink();
    app.get("/", (_, res) -> res.send("Hello World!"));
    app.listen(2037, true);
}

can be interacted with using curl:

curl -v -H "Connection: keep-alive" http://localhost:2037/ http://localhost:2037/

(some output lines omitted)

* Connection #0 to host localhost left intact
* Re-using existing connection with host localhost
* Connection died, retrying a fresh connect (retry count: 1)
* shutting down connection #0
* Issue another request to this URL: 'http://localhost:2037/'
* Connection #1 to host localhost left intact

Some clients, like node's fetch, do not try to reestablish the connection at all.

``` Uncaught TypeError: fetch failed at node:internal/deps/undici/undici:13185:13 at async REPL14:1:40 { [cause]: SocketError: other side closed ``` (NB: Node error does not occur on `master` because Weblink cannot find the "Connection" header, uppercase C. See #34)

When this patch is applied, the same curl command results in: (some lines omitted)

* Connection #0 to host localhost left intact
* Re-using existing connection with host localhost
* Connection #0 to host localhost left intact

"Connection: close" still works as well:

* shutting down connection #0
* shutting down connection #1