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: https://github.com/nodejs/node/pull/15650
Refs: https://github.com/nodejs/node/pull/13214#issuecomment-304049523


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>