`Attr.Parent()`/`ParentElement()` return incorrect values

`Attr` nodes return the element they are placed in as parent, but in a browser; the two values are nil.

The implementation was created as an `Attr` cannot be moved to another element.

```
// Throws, if `data-foo` is an attribute on elem2
elem1.setAttributeNode(elem2.getAttributeNode("data-foo"))

// While this is allowed
attr = elem2.getAttributeNode("data-foo")
elem2.removeAttributeNode(attr)
elem1.setAttributeNode(attr)
```

It was assumed that the information on the attribute was stored as the parent; but that was incorrect.

Start at Attr.Parent() and Attr.ParentElement() and compare their behavior with the browser semantics described in the issue. Done means both methods return nil for Attr nodes, including after an attribute is removed from one element and attached to another.

Written by the indexing model from the issue text.