来自我的Rails Web服务的HTTP响应如下，它正确指定Last-Modified和Etag以及Cache-Control标头。但是，像Firefox这样的客户端不会在请求中发回If-Modified-Since标头。
我在这里做错了什么？
HTTP/1.1 200 OK
Date: Mon, 10 Aug 2015 21:33:06 GMT
Server: Server
WWW-Authenticate: Negotiate oRQwEqADCgEAoQsGCSqGSIb3EgECAg==
X-Powered-By: Phusion Passenger (mod_rails/mod_rack) 3.0.12
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block
x-content-type-options: nosniff
Cache-Control: public
X-Request-Id: 0123c18f-956f-4a21-8584-501f9b340459
X-Runtime: 0.811858
Access-Control-Allow-Credentials: true
Access-Control-Allow-Methods: OPTIONS,GET
access-control-allow-headers: X-Requested-With,Content-type,Accept,Origin
access-control-max-age: 200
Etag: "fded13e6cc7dcec986e032f1f8d232cb-gzip"
Last-Modified: Fri, 07 Aug 2015 23:52:28 GMT
Status: 200
Vary: Accept-Encoding,User-Agent
Content-Encoding: gzip
Content-Length: 45764
Connection: close
Content-Type: application/json; charset=utf-8