[now dev] Respond with 404 once the server is shutting down (#2070)
[now dev] Respond with 404 once the server is shutting down (#2070)

In the case of a keep-alive connection from the web browser, and a page
continuing to send HTTP requests to the dev server, sometimes the server
never shuts down. So send a 404 response with `Connection: close` to
force the web browser to close the connection.
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