DateTimeInterface::ISO8601 Incompatibility with ISO8601 (#1576)

* Incompatibility DateTimeInterface::ISO8601

* Revert "Incompatibility DateTimeInterface::ISO8601"

This reverts commit 4b5264a8d8488c379c6ed3701ad971f6bec5bfb8.

* CodeGenerator DateTimeInterface::ATOM vs ISO8601

* Check timestampFormat Return

* Generate All

* Changelog fixed ATOM vs ISO8601

* iso8601 / timestampFormat

* Description of what is fixed

Author: ngourle

src/Generator/InputGenerator.php

@@ -562,6 +562,10 @@ private function stringify(string $variable, Member $member, string $part): stri
         switch ($shape->getType()) {
             case 'timestamp':
                 $format = strtoupper($shape->get('timestampFormat') ?? ('header' === $part ? 'rfc822' : 'iso8601'));
+                if ('ISO8601' === $format) {
+                    $format = 'ATOM';
+                }
+
                 if (!\defined('\DateTimeInterface::' . $format)) {
                     throw new \InvalidArgumentException('Constant "\DateTimeInterface::' . $format . '" does not exists.');
                 }