handle_conditional_get!

# File actionpack/lib/action_dispatch/http/cache.rb, line 191
        def handle_conditional_get!
          # Normally default cache control setting is handled by ETag middleware. But, if
          # an etag is already set, the middleware defaults to `no-cache` unless a default
          # `Cache-Control` value is previously set. So, set a default one here.
          if (etag? || last_modified?) && !self._cache_control
            self._cache_control = DEFAULT_CACHE_CONTROL
          end
        end