ETag: use body instead of @body since the method will always return a string

actionpack/lib/action_dispatch/http/cache.rb

@@ -89,7 +89,7 @@ def handle_conditional_get!
           if etag? || last_modified? || !@cache_control.empty?
             set_conditional_cache_control!
           elsif nonempty_ok_response?
-            self.etag = @body
+            self.etag = body
 
             if request && request.etag_matches?(etag)
               self.status = 304