Re: [dom] Supported property names for NamedNodeMap (#141)

I don't have deep knowlage around all ECMAScript stuff so I have to ask, in this example (test in Firefox):

```
<script>

 var newP = document.createElement("P");
 newP.setAttributeNS("www.test1.com", "A:new", "Test1");
 
 console.log(newP.attributes); // NamedNodeMap [ A:new="Test1" ]
 console.log(Object.getOwnPropertyNames(newP.attributes)); // Array [ "0", "A:new" ]
 console.log(newP.attributes.hasOwnProperty("A:new")); // false
 console.log(newP.attributes["A:new"]); // undefined
 console.log(Object.getOwnPropertyDescriptor(newP.attributes, "A:new")); // undefined
 
</script>
```
Returning "A:new" by `getOwnPropertyNames()` when other methods return false/undefined is correct/intend?

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https://github.com/whatwg/dom/issues/141#issuecomment-169197535