etag header is in Rack, so use it's response methods

Rack implements the Etag header manipulation methods, so we can use
those instead of ours.

Gemfile.lock

@@ -29,7 +29,7 @@ GIT
 
 GIT
   remote: git://github.com/rack/rack.git
-  revision: c617ea99c12a5bfe026e00476ff37e714e01891a
+  revision: e2a8388d59bbad0cb867150185770ad76e80d31a
   branch: master
   specs:
     rack (2.0.0.alpha)

actionpack/lib/action_dispatch/http/cache.rb

@@ -82,19 +82,15 @@ def date=(utc_time)
 
         def etag=(etag)
           key = ActiveSupport::Cache.expand_cache_key(etag)
-          set_header ETAG, %("#{Digest::MD5.hexdigest(key)}")
+          super %("#{Digest::MD5.hexdigest(key)}")
         end
 
-        def etag
-          get_header ETAG
-        end
-        alias :etag? :etag
+        def etag?; etag; end
 
       private
 
         DATE          = 'Date'.freeze
         LAST_MODIFIED = "Last-Modified".freeze
-        ETAG          = "ETag".freeze
         CACHE_CONTROL = "Cache-Control".freeze
         SPECIAL_KEYS  = Set.new(%w[extras no-cache max-age public must-revalidate])