Issue title: Zend\Config\Config, count wrong after merge
 Issue body: 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());
}
```

 Comments: 
 Comment 0: Closed.  Didn't realize a pull request creates an issue.  Moved to https://github.com/zendframework/zf2/issues/3300

{'source': 'GithubIssues', 'repo_name_with_owner': 'zendframework/zendframework', 'issue_id': 9547667, 'repo_stars': 5581.0}