MDL-65641 forum: Make forum post form to honor existing MOODLE_FORMAT

Since the night of the times (MDL-16698) we have been performing
that simple empty($post->messageformat) check to decide if the
preferred editor format should be used, instead of any existing format.

Problem is that FORMAT_MOODLE == 0, so the condition evaluates
to to true and then the preferred editor takes the baton when it should
not. Exiting format must win always!

So this just makes the condition a little better, checking if the
attribute is set and that it's numerical (accepting both n and 'n').

That covers the FORMAT_MOODLE case and any other valid format. Only if the
conditions aren't met the preferred editor format takes the baton. Only
then.

mod/forum/post.php

@@ -737,7 +737,8 @@ $mformpost->set_data(
         'subject' => $post->subject,
         'message' => array(
             'text' => $currenttext,
-            'format' => empty($post->messageformat) ? editors_get_preferred_format() : $post->messageformat,
+            'format' => !isset($post->messageformat) || !is_numeric($post->messageformat) ?
+                editors_get_preferred_format() : $post->messageformat,
             'itemid' => $draftideditor
         ),
         'discussionsubscribe' => $discussionsubscribe,