Description:
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For some reasons headers are not outputted when script is terminated with set_exception_handler().

I was not able to isolate the issue, simple test below does not have it, but the complex example below is repeatable across multiple servers and PHP versions (7.2 - 7.4.4).

Test script:
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# this works even without flush()
header('Content-Type: application/json', true);
echo json_encode('test');
// flush();
set_exception_handler(function (Throwable $t) {return true;});
throw new \Exception();

# but in this code flush() is required, otherwise "text/html; charset=UTF-8" is outputted even if this exact content-type (with charset) is not outputted in this project at at (it has to come from PHP)
https://github.com/atk4/ui/pull/1051/commits/423251f13e515a0dadaed548abe17989cfbf0f33#diff-58980ac23a3a9a0b92abd9805eed1e78R227
CI results prior fixing with flush(): https://github.com/atk4/ui/runs/556304096?check_suite_focus=true#step:9:50

It can be easily reproduced locally when you clone https://github.com/mvorisek/atk4_ui/commit/46411692933745ef7b715c608363393afc803b85 and open URL ./demos/sticky2.php?__atk_reload=atk_admin_button . Issue can be reproduced with fast-cgi and internal php web server.

Expected result:
----------------
Headers, ie. content-type in the example, are always sent even if the execution is immediatelly terminated.

Actual result:
--------------
Headers are not outputed.