Make sure subscriptions are sent after reconnects

Fixes a bug where a hpfeed client would stop receiving data after a
reconnect.

To reproduce start the client:
```shell
hpfeeds-client -i XXXXXXX -s YYYYYYYYY --host hpfriends.honeycloud.net -p 20000 -c dionaea.capture subscribe
```
Afterwards somehow kill the connection:
```shell
tcpkill -i eth0 host hpfriends.honeycloud.net
CTRL-C
```
After those steps the client will reconnect but not resend subscriptions.