Rails4: fix PreviewsControllerTest assert_template failures

<pre>
  1) Failure:
PreviewsControllerTest#test_preview_issue_notes [test/functional/previews_controller_test.rb:44]:
expecting <"preview"> but rendering with <["previews/issue"]>
</pre>

git-svn-id: http://svn.redmine.org/redmine/trunk@12879 e93f8b46-1217-0410-a6f0-8f06a7374b81