Fix Bug: Init When Textarea Starts with Comment

Closes issue #4409

It looks like the check for "bogus-node" was calling getAttribute on the
node, but comment DOM nodes don't have attributes, so this check was
failing.  This commit adds a test for the problem (the test will throw
an exception on failure) and a fix for the problem.

Link to issue:  http://www.tinymce.com/develop/bugtracker_view.php?id=4409
Other possibly related issues: 

http://www.tinymce.com/develop/bugtracker_view.php?id=4340
http://www.tinymce.com/develop/bugtracker_view.php?id=5925