Alternative base URL for PURGE requests does not work when "Include base URL in expires" is enabled

When Expire is configured with the "Include base URL in expires" option enabled, the alternative base URL for PURGE requests introduced in #2644756: Purge does not work if domain is locally binded to 127.0.0.1 → is not used. When trying the build the alternative URL for the purge request, an absolute $original_url is an absolute URL and thus url() does not change the domain.