[dom] cloneNode underspecified for attribute copying (#30)

The spec says:

> Copy the following from node to copy, depending on the type of node: 
> ...
> (If Element) Its namespace, namespace prefix, local name, and its attribute list.

Copying the attribute list is underspecified. It seems like it could mean any of the following:

1. Set _copy_@[[attributeList]] to _node_@[[attributeList]], i.e., maintain a pointer, so that manipulating either element will affect the other.
2. Loop through _node_@[[attributeList]] and append each attribute to _copy_@[[attributeList]]. In this case, adding or removing attributes from _copy_ will not affect _node_, but updating existing ones will.
3. Loop through _node_@[[attributeList]] and create new `Attr` objects to append to _copy_@[[attributeList]].

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https://github.com/whatwg/dom/issues/30