Capture socket in response finish callback closure directly rather than relying on request object

[mikegreiling]

In certain circumstances, a request object's socket will be disassociated or destroyed prior to the response's finish event firing. In this case, req.socket will be null. This change ensures that the .on('finish') closure will reference the socket directly rather than referencing it through the request.

uncaughtException: TypeError: Cannot set properties of null (setting '_isIdle')
    at ServerResponse.<anonymous> (/Users/admin/Projects/home-portal/node_modules/http-graceful-shutdown/lib/index.js:160:26)
    at ServerResponse.emit (node:events:531:35)
    at onFinish (node:_http_outgoing:1031:10)
    at callback (node:internal/streams/writable:764:21)
    at afterWrite (node:internal/streams/writable:708:5)
    at afterWriteTick (node:internal/streams/writable:694:10)
    at process.processTicksAndRejections (node:internal/process/task_queues:81:21)