Support DateTimeImmutable in Timestamp API Fields (#2241)

Author: chrisguitarguy

src/Api/TimestampShape.php

@@ -24,7 +24,7 @@ public function __construct(array $definition, ShapeMap $shapeMap)
      */
     public static function format($value, $format)
     {
-        if ($value instanceof \DateTime) {
+        if ($value instanceof \DateTimeInterface) {
             $value = $value->getTimestamp();
         } elseif (is_string($value)) {
             $value = strtotime($value);

tests/Api/TimestampShapeTest.php

@@ -22,7 +22,10 @@ public function formatProvider()
             [$t, 'rfc822', 'Tue, 05 Jan 1999 00:00:00 GMT'],
             [new \DateTime('january 5, 1999'), 'unixTimestamp', '915494400'],
             [new \DateTime('january 5, 1999'), 'iso8601', '1999-01-05T00:00:00Z'],
-            [new \DateTime('january 5, 1999'), 'rfc822', 'Tue, 05 Jan 1999 00:00:00 GMT']
+            [new \DateTime('january 5, 1999'), 'rfc822', 'Tue, 05 Jan 1999 00:00:00 GMT'],
+            [new \DateTimeImmutable('january 5, 1999'), 'unixTimestamp', '915494400'],
+            [new \DateTimeImmutable('january 5, 1999'), 'iso8601', '1999-01-05T00:00:00Z'],
+            [new \DateTimeImmutable('january 5, 1999'), 'rfc822', 'Tue, 05 Jan 1999 00:00:00 GMT']
         ];
     }