http: do not blindly destroy UNIX domain sockets

`Connection: keep-alive` is now properly supported when making client
connections to UNIX domain sockets so `request.abort()` should not
blindly destroy the underlying socket.

PR-URL: nodejs/node#15650
Refs: nodejs/node#13214 (comment)
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>

lib/_http_client.js

@@ -663,7 +663,7 @@ ClientRequest.prototype.onSocket = function onSocket(socket) {
 function onSocketNT(req, socket) {
   if (req.aborted) {
     // If we were aborted while waiting for a socket, skip the whole thing.
-    if (req.socketPath || !req.agent) {
+    if (!req.agent) {
       socket.destroy();
     } else {
       socket.emit('free');

test/parallel/test-http-client-abort-keep-alive-queued-unix-socket.js

@@ -0,0 +1,38 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+let socketsCreated = 0;
+
+class Agent extends http.Agent {
+  createConnection(options, oncreate) {
+    const socket = super.createConnection(options, oncreate);
+    socketsCreated++;
+    return socket;
+  }
+}
+
+const server = http.createServer((req, res) => res.end());
+
+const socketPath = common.PIPE;
+common.refreshTmpDir();
+
+server.listen(socketPath, common.mustCall(() => {
+  const agent = new Agent({
+    keepAlive: true,
+    maxSockets: 1
+  });
+
+  http.get({ agent, socketPath }, (res) => res.resume());
+
+  const req = http.get({ agent, socketPath }, common.mustNotCall());
+  req.abort();
+
+  http.get({ agent, socketPath }, common.mustCall((res) => {
+    res.resume();
+    assert.strictEqual(socketsCreated, 1);
+    agent.destroy();
+    server.close();
+  }));
+}));