add support for DateTimeImmutable

Adds support for DateTimeImmutable by using the shared DateTimeInterface
which both this and DateTime implement.

src/Spout/Common/Helper/CellTypeHelper.php

@@ -61,7 +61,7 @@ class CellTypeHelper
     public static function isDateTimeOrDateInterval($value)
     {
         return (
-            $value instanceof \DateTime ||
+            $value instanceof \DateTimeInterface ||
             $value instanceof \DateInterval
         );
     }

tests/Spout/Common/Helper/CellTypeHelperTest.php

@@ -90,6 +90,7 @@ class CellTypeHelperTest extends TestCase
     public function testIsDateOrInterval()
     {
         $this->assertTrue(CellTypeHelper::isDateTimeOrDateInterval(new \DateTime()));
+        $this->assertTrue(CellTypeHelper::isDateTimeOrDateInterval(new \DateTimeImmutable()));
         $this->assertTrue(CellTypeHelper::isDateTimeOrDateInterval(new \DateInterval('P1D')));
 
         $this->assertFalse(CellTypeHelper::isDateTimeOrDateInterval(true));