Opera 9.51 does not cache ajax?

We usually try to avoid caching ajax requests, but there are times when it is necessary. For example, if ajax is used to load scripts. All modern browsers do an excellent job with caching static files, but Opera under no circumstances wants to cache ajax requests, although everything else is cached normally.

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I conducted a test: made a file test.php:
header ("Cache-control: public");
header ("Cache-control: max-age = 1800");
echo rand ();

I downloaded this file via ajax. If the number does not change, then the file is cached.
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IE7 and FF3 cache iron, the response from the server does not change.
Safari caches, but when you click on the "Update" button resets the cache.
Opera does not cache the request under any circumstances.

Someone tell me how to make her do it?

UPD: I forgot to say that the dojo has the same problems - in the opera, each page is loaded for a minute.