Use root URL to check against resource URL

[nlemoine]

Hi,

Using site_url to get the "root" URL is always wrong. site_url is the path to WordPress. Indeed, most of the time (regular WP structure) home_url == site_url. But it can sometimes be different. In my case (using Bedrock or any WordPress-in-a-subdirectory structure type), site_url is https://domain.com/wp. This kind of setup is pretty popular among developers.

As pointed out in #19, multi language plugins such Polylang or WPML change the value of home_url. That's why this PR parses home URL to extract the root domain only.

Maybe the comparison should be more accurate. An edge case like https://externaldomain.com/domain.com.css would match the check. But yes, it is far-fetched and very unlikely to happen 😄