Cell indexes not being respected when rendering row

Fixes #682
When calling `Row::setCellIndex`, it's possible to create a Row with holes.
Instead of iterating over existing cells of a Row, we should instead use the cell indexes (from 0 to max cell index).

tests/Spout/Common/Entity/RowTest.php

@@ -78,10 +78,24 @@ class RowTest extends \PHPUnit\Framework\TestCase
         $row = new Row([], null);
         $cellMock = $this->getCellMock();
         $row->setCellAtIndex($cellMock, 3);
+
         $this->assertEquals($cellMock, $row->getCellAtIndex(3));
         $this->assertNull($row->getCellAtIndex(10));
     }
 
+    /**
+     * @return void
+     */
+    public function testSetCellAtIndex()
+    {
+        $row = new Row([], null);
+        $cellMock = $this->getCellMock();
+        $row->setCellAtIndex($cellMock, 1);
+
+        $this->assertEquals(2, $row->getNumCells());
+        $this->assertNull($row->getCellAtIndex(0));
+    }
+
     /**
      * @return void
      */