Prevent user from overwritting required code

As users advance in tutorial, they're told to change the previously created ArticlesTable.php file and if it does not contains initialize() with Timestamp behaviour, it'll cause the views containing <?= $article->created->format(DATE_RFC850) ?> (index.ctp and view.ctp) to fail as users creates new articles that will have 'created' as null. Better to maintain the function there as readers could copy and paste like I just did and broke the blog index and view.
Merge pull request #1533 from Hikkijp/patch-2

Prevent user from overwritting required code