"formEnctype" and "formMethod" getters on <input> and <button> are wrong

[eefriedman]

Test in tests/wpt/web-platform-tests/html/dom/reflection-forms.html. Spec URL: https://html.spec.whatwg.org/multipage/forms.html#attr-fs-enctype . Note specifically the difference between the "invalid value default" and the "missing value default". (The "missing value default" only applies if the attribute is not present, i.e. hasAttribute() returns false.)