implement Cell:isDate() for unification

tests/Spout/Common/Entity/CellTest.php

@@ -54,6 +54,15 @@ class CellTest extends \PHPUnit\Framework\TestCase
         $this->assertTrue((new Cell(false))->isBoolean());
     }
 
+    /**
+     * @return void
+     */
+    public function testCellTypeDate()
+    {
+        $this->assertTrue((new Cell(new \DateTime()))->isDate());
+        $this->assertTrue((new Cell(new \DateInterval('P2Y4DT6H8M')))->isDate());
+    }
+
     /**
      * @return void
      */