Webfinger resource identifier and response are using internal domain name

From https://github.com/xwiki-contrib/application-activitypub/issues/40

Using the URL scheme:

http://serverUrl/webfinger/?resource=\{actorId}@{domain}

The {{

}} only worked with the internal LAN domain and not the public reverse-proxied domain (serverUrl in this case). Using the public domain returned "No user found for the given domain". The URLs in the Webfinger JSON data that was returned also were all based on the internal non-proxied domain, which will not work for external servers to look up AP Actor information.