[RobertoPrevato]

• Add a vary parameter to the cache_control decorator and to
  CacheControlMiddleware, to add field names to the Vary response header
  (e.g. @cache_control(max_age=60, vary=["Accept-Language"])). Names already
  listed in the response Vary header are not repeated (case-insensitive), and
  nothing is added when the response has Vary: *.
• Fix GzipMiddleware to add Vary: Accept-Encoding to responses that are
  eligible for compression (also when not compressed for a given request), so
  that shared caches and CDNs do not serve a compressed response to clients that
  don't support it, or vice versa.