Add missing <fieldset> attributes

[alexisfontaine]

https://html.spec.whatwg.org/#the-fieldset-element

[benwis]

@alexisfontaine This seems fine, did you try to use these with attribute spreading? attr:name=blah? Still happy to merge it, just curious

[alexisfontaine]

Attributes spreading does works on a component wrapping a native <fieldset>. Although I prefer view!(<fieldset {..disabled(true)} />), or the equivalent builder syntax fieldset().add_any_attr(disabled(true)).

[benwis]

Thanks!