[whatwg/dom] MutationObserver: observing attribute changes fires even if the value didn't change. (#520)

If I call `setAttribute(sameValueAsBefore)` on an element, passing the same value as the attribute already has, the MutationObserver instance still register a mutation. This is my observer:

```js
const attributeChangedObserver = new MutationObserver( records => {
    console.log( ' ---------- attribute change records', records )
} )

attributeChangedObserver.observe( element, {
    attributes: true,
    attributeOldValue: true,
    attributeFilter: observedAttributes
} )
```

This seems strange, because the attribute value didn't change. Why is it this way?

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