Add support for Comment in node building

I have a document fragment in a string, was calling ```template/html->nodes``` on it, but part of that string contains a ```<!-- comment -->```. Dommy was throwing an exception as there was no PElement for a comment and so blew up with: _Don't know how to make node from: #<[object Comment]>_. 

Minor code change attached to add that, and a test to verify it worked.