[DOMCore] ownerDocument of a Document object

Both Firefox and Chrome make the ownerDocument of a Document node null.

The June 3rd spec just says:
> The |ownerDocument| attribute must return the |Document 
> <http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#document>| 
> node that the context object 
> <http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#context-object> 
> is associated with, or null if there is none. 
The meaning of this depends on whether a node is "associated with" itself.

But the (new, I think) algorithm to ensure that two node are in the same 
document requires document.ownerDocument === document, or it will fail 
in the case of inserting a doctype into a document.

     David