Wikia.InstantGlobals doesn't supply the proper Last-Modified header

https://wikia-inc.atlassian.net/browse/PLATFORM-298

Emit ``Last-Modified`` response header with the current timestamp. It will cause CDN to update the cache entry on every miss.

@gabrys / @Wikia/platform-team 
this may introduce the unnecessary content difference for "startup" ResourceLoader module that contains all timestamps of all modules, did we consider that here?