Ruby on Rails: how to set content expiration for 15 minutes if it is an AJAX request

Is the following a good form of detecting AJAX request and setting expiration for 15 minutes so that the same GET will not require any network traffic?

# in controller
if request.xhr?
  response.headers['Expires'] = (Time.now + 15.minutes).httpdate
  response.headers['Cache-Control'] = ''   # override any no-cache, must-revalidate
end

Update: I found a shorter alternative, which is expires_in

# in controller
if request.xhr?
  expires_in(15.minutes)
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although, after that, the header becomes:

Cache-Control: max-age=900, private

which used to be

Expires: Tue, 13 Jul 2010 00:59:47 GMT

when it was the earlier version. But note that Cache-Control is for HTTP/1.1 and even a couple years ago, it was supported by 99% of the browsers, as mentioned in the High Performance Website book.

Ok, since there is no answer yet, I am going to use the one in the update above:

if request.xhr?
  expires_in(15.minutes)
end

I tried it and it works well.