CodegenRust fixes for NamedGetter
Added Element::set_custom_attribute
Implement HTMLElement.dataset

Make DOMStringMap use related Element's custom attributes values.
Avoid duplicated code
Update web-platform-tests results
auto merge of #4038 : brunoabinader/servo/dataset, r=Ms2ger

Spec: https://html.spec.whatwg.org/multipage/dom.html#dom-dataset

Closes #2974.