Guard against no subscription in on_unsubscribe
Guard against no subscription in on_unsubscribe

Don't assume subscribe_to has been called
on a subscriber when we're unsubscribing it.
ksw2599/torquebox,mje113/torquebox,mje113/torquebox,torquebox/torquebox,torquebox/torquebox,torquebox/torquebox-release,torquebox/torquebox-release,torquebox/torquebox,torquebox/torquebox-release,ksw2599/torquebox,torquebox/torquebox-release,torquebox/torquebox,mje113/torquebox,ksw2599/torquebox,samwgoldman/torquebox,vaskoz/torquebox,mje113/torquebox,ksw2599/torquebox,vaskoz/torquebox,samwgoldman/torquebox,samwgoldman/torquebox,samwgoldman/torquebox,vaskoz/torquebox,vaskoz/torquebox