Missing tags after HUP signal

Hi team,

Tags configured in a serf node are missed after send a HUP signal. I've a tags-file configured to use persistent tags, but seems to be ignored after an HUP signal. However, tags doesn't missed after restarting serf process.

```
[root@localhost ~]# cat /etc/serf/00first.json
{"bind":"10.0.203.58","tags_file":"/etc/serf/tags","node_name":"rb-jqlhlwo9y6"}

[root@localhost ~]# serf members
rb-jqlhlwo9y6 10.0.203.58:7946 alive

[root@localhost ~]# serf tags -set newTag=testing
Successfully updated agent tags

[root@localhost ~]# serf members
rb-jqlhlwo9y6 10.0.203.58:7946 alive newTag=testing

[root@localhost ~]# kill -SIGHUP 17063

[root@localhost ~]# serf members
rb-jqlhlwo9y6 10.0.203.58:7946 alive
```
I'm using 0.8.0 version.

Best regards