Zend\Config - Fix count after merge

The following test fails.  The count never gets updated after a merge

**ZendTest\Config\ConfigTest.php**
```php
public function testCountAfterMerge()
{
      $data = new Config($this->toCombineB);
      $data->merge(
          new Config($this->toCombineA)
      );
      $this->assertEquals(count($data->toArray()), $data->count());
}
```