Support Cache-Control headers

[twm]

Some feeds don't provide Last-Modified or Etag headers, for example:

$ curl -vv https://www.imperialviolet.org/iv-rss.xml
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0*   Trying 159.203.111.115...
...
> GET /iv-rss.xml HTTP/1.1
> Host: www.imperialviolet.org
> User-Agent: curl/7.47.0
> Accept: */*
> 
< HTTP/1.1 200 OK
< Cache-Control: max-age=900, public
< Content-Length: 152707
< Content-Type: application/xml
< Date: Mon, 24 Jul 2017 08:39:50 GMT
....

That's kinda lame, but hey, a feed reader is the best way to discover the seedy underbelly half-assed portion of the Internet.