JEditor errors on addCustomTag() call when using raw document

### Tracker
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33760&start=0

### Description
addCustomTag is not a valid method for JDocumentRaw and generates an error if the URL contains "format=raw", which I use for displaying forms within an AJAX dialog (the form contains an editor widget).

### Code by Dan Cogliano