Add some extra headers

[vitr]

see igrigorik/ga-beacon#47
We're pretty confident that the image caching works. If you're having issues with something like a CI badge, make sure the image has the Cache-Control: no-cache header, and either Expires, Last-Modified or Etag

[vitr]

I don't think it helps at all

[ArtemGr]

I don't think it helps at all

So, have you tried it?

BTW, I've delved a bit deeper into specifics there: igrigorik/ga-beacon#47 (comment).