fix: Unref socket on response (#14)

.changeset/slimy-seas-type.md

···
+---
+'fetch-nodeshim': patch
+---
+
+Unref the incoming socket when the timeout is disabled, to prevent body streams that never start from keeping processes alive

src/fetch.ts

···
 
     outgoing.on('response', incoming => {
       incoming.setTimeout(0); // Forcefully disable timeout
+      incoming.socket.unref();
 
       const init = {
         status: incoming.statusCode,