For bans, the Host header wouldn't need to be set, but it doesn't hurt to have it either.

I set the host from `$this->host` exactly when the URL is a path (not an absolute URL), so it doesn't contain a hostname. As we need a hostname for purge and refresh, we use the default one from `$this->host`. Currently, the `refresh` and `purge` methods do not set a Host header, so we don't need to check here. But perhaps it would be useful for users extending the Varnish class, setting their own Host header and then calling our `queueRequest` method?