Bug 1063364. imgLoader::SetHasProxies/SetHasNoProxies should operate on imgRequests/imgCacheEntrys not URIs. r=seth

There can be more than one imgRequest alive for any given URI, so when we tell the imgLoader that we have/do not have proxies we should be specific which imgRequest/imgCacheEntry this applies to so we don't change the wrong imgRequest/imgCacheEntry.