fix: headers before flushing (#1622)

* add tests

* fix test

* attempt to send headers when flushing

* Update testdata/only-headers.php

Co-authored-by: Kévin Dunglas <kevin@dunglas.fr>

---------

Co-authored-by: Kévin Dunglas <kevin@dunglas.fr>

testdata/only-headers.php

@@ -0,0 +1,11 @@
+<?php
+
+require_once __DIR__.'/_executor.php';
+
+return function () {
+    header('Content-Type: application/json');
+    header('HTTP/1.1 204 No Content', true, 204);
+
+    echo '{"status": "test"}';
+    flush();
+};