importNode always returns XMLNode instead of right node type

importNode always returns createXMLReferenceOrThrow(XMLNode, ...), so many methods will return wrong type with elements from another document.
```
let elem = doc1.root().addChild(doc2.createElement("elem"))
// elem.getAttribute and so on does not exist!
console.log(elem.getAttribute == undefined)
```
A solution could be to use refToNodeType() instead to return correct type.

See also #653 
