pingCommand doesn't seem to be working as documented
This is my configuration:

```
{
    "accessory": "NetworkDevice",
    "name": "My PC",
    "ip": "10.0.0.10",
    "pingInterval": 5,
    "pingCommand": "curl -m 2 -s http://10.0.0.10:5001",
    "timeout": 2,
    "wakeGraceTime": 10,
    "shutdownCommand": "curl -s http://10.0.0.10:5001/.../suspend",
    "log": true,
    "logPinger": true
}
```

I am running homebridge-wol v3.2.4 and the pingCommand doesn't seem to be working properly. It always considers the machine to be running even when it's not.

I've tried running the curl command from another machine and it returns a code higher than 0 when sleeping and the code 0 when up:

Up:

    $ curl -m 2 -s 10.0.0.10:5001
    Ok
    $ echo $?
    0

Down:

    $ curl -m 2 -s 10.0.0.10:5001
    $ echo $?
    7

On that 5001 port I am running a small web server that returns the output "Ok". This is [Karpach Remote Shutdown PC](https://github.com/karpach/remote-shutdown-pc).

Also, on the Log:

```
[2019-9-29 19:20:04] Homebridge is running on port 51826.
[2019-9-29 19:20:04] [My PC] Pinger saw device online; setting to Online.
[2019-9-29 19:20:04] [My PC] NetworkDevice "My PC" (10.0.0.10) went from status "Offline" to "Online"
[2019-9-29 19:20:33] [My PC] NetworkDevice shutdown cycle started for "My PC" (10.0.0.10)
[2019-9-29 19:20:33] [My PC] Stopping pinger
[2019-9-29 19:20:33] [My PC] Starting pinger again in 15000 milli seconds
[2019-9-29 19:20:33] [My PC] Attempting to shut down "My PC" (10.0.0.10) using "curl -m 2 -s http://10.0.0.10:5001/.../suspend"
[2019-9-29 19:20:33] [My PC] NetworkDevice "My PC" (10.0.0.10) went from status "Online" to "Shutting Down"
[2019-9-29 19:20:35] [My PC] An error occured while trying to shut down "My PC" (10.0.0.10): Error: Command failed: curl -m 2 -s http://10.0.0.10:5001/.../suspend

[2019-9-29 19:20:35] [My PC] Stopping pinger
[2019-9-29 19:20:35] [My PC] Starting pinger at an interval of 5000 milli seconds
[2019-9-29 19:20:35] [My PC] Pinger saw device online; setting to Online.
[2019-9-29 19:20:35] [My PC] NetworkDevice "My PC" (10.0.0.10) went from status "Shutting Down" to "Online"
```

The shutdown command fails as it should because the machine is already offline. However, there's no log on the ping command failing.