Serf leaves but client returns errors anyways

I am hitting an edge-case on the `centos:6` Docker container, when executing `serf leave -rpc-addr=127.0.0.1:7373` effectively triggers the `leave` event on the agent, but then the client returns with the following error:

```
Error leaving: client closed
```

Although the agent's logs properly confirm the graceful shutdown:

```
==> Starting Serf agent...
==> Starting Serf agent RPC...
==> Serf agent running!
Node name: 'f71dbac7f660'
Bind addr: '0.0.0.0:7946'
RPC addr: '127.0.0.1:7373'
Encrypted: false
Snapshot: false
Profile: lan

==> Log data will now stream in as it occurs:

2016/08/08 02:13:35 [INFO] agent: Serf agent starting
2016/08/08 02:13:35 [INFO] serf: EventMemberJoin: f71dbac7f660 172.17.0.2
2016/08/08 02:13:36 [INFO] agent: Received event: member-join
2016/08/08 02:13:41 [INFO] agent.ipc: Accepted client: 127.0.0.1:43884
2016/08/08 02:13:41 [INFO] agent.ipc: Graceful leave triggered
2016/08/08 02:13:41 [INFO] agent: requesting graceful leave from Serf
2016/08/08 02:13:41 [INFO] serf: EventMemberLeave: f71dbac7f660 172.17.0.2
2016/08/08 02:13:41 [INFO] agent: requesting serf shutdown
2016/08/08 02:13:41 [INFO] agent: shutdown complete
```

This can be replicated by starting the Serf agent like:

``` shell
serf agent > serf.log 2>&1 &
```

And then executing:

``` shell
serf leave -rpc-addr=127.0.0.1:7373
```

Sometimes it will work properly, but often the client will return the error.