reflect href on buttons (#817)

src/components/button/button.ts

@@ -109,7 +109,7 @@ export default class WaButton extends WebAwesomeFormAssociatedElement {
   @property({ reflect: true }) value: string | null = null;
 
   /** When set, the underlying button will be rendered as an `<a>` with this `href` instead of a `<button>`. */
-  @property() href = '';
+  @property({ reflect: true }) href = null;
 
   /** Tells the browser where to open the link. Only used when `href` is present. */
   @property() target: '_blank' | '_parent' | '_self' | '_top';