auth_basic lines appearing in SSL vhost header when they shouldn't

I had nginx crashing on startup because there were some malformed lines about `auth_basic` in the configuration for a vhost using SSL, even though I hadn't specified anything to do with basic auth in Puppet.

Turns out that [`vhost_header.erb`][erb] and `vhost_ssl_header.erb` use a different syntax to test if `@auth_basic` is set, so I changed the latter to match the former, and it worked.

[erb]: https://github.com/jfryman/puppet-nginx/blob/master/templates/vhost/vhost_header.erb#L9-L14

Possibly related to #100.