http.Agent: idle sockets throw unhandled ECONNRESET

Please see this code: https://gist.github.com/thorn0/0812cdb7eb12b2348337
That's what it outputs for me (Node 4.2.1 and 5.0.0, Windows 7 x64):

```
1.108s: Complete
121.193s: uncaughtException
Error: read ECONNRESET
    at exports._errnoException (util.js:874:11)
    at TCP.onread (net.js:544:26)
```

It's reproduced with most servers that run Microsoft IIS.
