lint: fix lint errors
trunk-tracking: update from r3827 to r3852

* port EnrollExperiment tests.
* tricky changes to get gzip+ipro working properly.
* reduce code duplication by calling copy_response_headers_from_ngx instead
  of reimplementing it.
* port Vary: Accept test changes.
Merge pull request #639 from pagespeed/jefftk-trunk-tracking

trunk-tracking: update from r3827 to r3852