remove `@etag` ivar

we're storing the value in the headers hash, so lets just store the
value in one place.

actionpack/lib/action_dispatch/http/cache.rb

@@ -51,8 +51,7 @@ def fresh?(response)
       end
 
       module Response
-        attr_reader :cache_control, :etag
-        alias :etag? :etag
+        attr_reader :cache_control
 
         def last_modified
           if last = get_header(LAST_MODIFIED)
@@ -84,9 +83,13 @@ def date=(utc_time)
 
         def etag=(etag)
           key = ActiveSupport::Cache.expand_cache_key(etag)
-          @etag = %("#{Digest::MD5.hexdigest(key)}")
-          set_header ETAG, @etag
+          set_header ETAG, %("#{Digest::MD5.hexdigest(key)}")
+        end
+
+        def etag
+          get_header ETAG
         end
+        alias :etag? :etag
 
       private
 
@@ -124,7 +127,6 @@ def cache_control_headers
 
         def prepare_cache_control!
           @cache_control = cache_control_headers
-          @etag = get_header ETAG
         end
 
         def handle_conditional_get!