问rails 4 http缓存返回200 iso 304，即使使用相同的ETag和last_modified

EN

def index
  ...#some code

  # HTTTP caching:
  last_mod = @scraps.order("updated_at").last.updated_at
  user = current_user ? current_user.id : 0

  fresh_when etag: user.to_s, last_modified: last_mod, public: false
end

HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
Vary: Accept-Encoding
Status: 200 OK
Last-Modified: Sun, 23 Jul 2017 20:40:53 GMT
Cache-Control: max-age=0, private, must-revalidate
ETag: W/"6e92592bdb6c3cf610020e2b076e64b4"
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block
X-Content-Type-Options: nosniff
X-Runtime: 3.187090
X-Request-Id: c698c0c6-8a0d-44ba-8ca9-3f162b766478
Date: Mon, 24 Jul 2017 14:49:38 GMT
Set-Cookie: ... [edited out]; path=/; HttpOnly
X-Powered-By: Phusion Passenger 5.0.30
Server: nginx/1.10.1 + Phusion Passenger 5.0.30
Content-Encoding: gzip

GET /scraps?page=3&price_max=100&price_min=0&producer=silk+scraps HTTP/1.1
Host: www.picture-scraps.com
Connection: keep-alive
Accept: text/html, application/xhtml+xml, application/xml
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.115 Safari/537.36
X-XHR-Referer: https://www.picture-scraps.com/scraps?page=4&price_max=100&price_min=0&producer=silk+scraps
Referer: https://www.picture-scraps.com/scraps?page=4&price_max=100&price_min=0&producer=silk+scraps
Accept-Encoding: gzip, deflate, br
Accept-Language: nl-NL,nl;q=0.8,en-US;q=0.6,en;q=0.4,af;q=0.2
Cookie: ... [edited out]
If-None-Match: W/"6e92592bdb6c3cf610020e2b076e64b4"
If-Modified-Since: Sun, 23 Jul 2017 20:40:53 GMT