Changed host from 'HTTP_HOST' to 'SERVER_NAME'

I ran into an issue where Url::get() was including the port twice `http://example.com:1180:1180/`. The value of `$_SERVER['HTTP_HOST']` is taken directly from the `Host: HTTP` request header and it appears some browsers (Chrome, in my case) include the port. `$_SERVER['SERVER_NAME']` gets its value from the virtual host config without the port.

Let me know if there is a better way of fixing this and I will amend the PR.