use DOM API instead of attribute for "selected"

https://developer.mozilla.org/en-US/docs/Web/API/HTMLOptionElement

The Option element exposes a "selected" getter that works even when the "selected" attribute is not set, such as when the element is created by React or manipulated. Fixes react-quill#175.

ui/picker.js

@@ -43,7 +43,7 @@ class Picker {
     [].slice.call(this.select.options).forEach((option) => {
       let item = this.buildItem(option);
       options.appendChild(item);
-      if (option.hasAttribute('selected')) {
+      if (option.selected === true) {
         this.selectItem(item);
       }
     });