BAC-1319: Make resource loader output X-Served-by header

```
$ curl -I "http://poznan.macbre.wikia-dev.com/__load/-/cb%3D123456%26debug%3Dfalse%26lang%3Dpl%26only%3Dscripts%26skin%3Doasis/wikia.ext.abtesting" -s | grep -E "X-Served-By|X-Backend-Response-Time"
X-Backend-Response-Time: 0.149
X-Served-By: dev-macbre, cache-s24-SJC, cache-fra1227-FRA
```
ResourceLoader will respond with the same set of headers as added in #2587 

@prein / @michalroszka