getDOMNodeID: Don't return .id on random objects

If you defined a global named `id` (a horrible name, I know) then
getDOMNodeID(window) would return that object. Since only DOM nodes can
have IDs, this should be a noop change otherwise.

Test Plan:
Verified that document.documentElement and document.body both support
getAttributeNode properly in latest Chrome and in IE8.
Merge pull request #61 from spicyj/getdomnodeid

getDOMNodeID: Don't return .id on random objects