trunk-tracking: update from r3784 to r3811
The big change here is Vary: headers, which are no longer being stripped from the pagespeed response.  Take a look; it might be we want to be smart about merging these headers with the original request's Vary: headers.
Merge pull request #629 from pagespeed/jmaessen-trunk-tracking

trunk-tracking: update from r3784 to r3811