Followup r94375; Use PROTO_RELATIVE so that when $wgServer is a protocol relative...

Followup r94375; Use PROTO_RELATIVE so that when $wgServer is a protocol relative url we don't potentially polute the cache if getFullURL is called by an extension that outputs to cache. getFullURL includes a #fragment so it's more likely that extensions outputting urls in situations that don't support protocol relative urls are using getLocalURL and expanding it.