Make sure call on shouldUpdate works for mixins and use value instead of id.

tp-dropdown.js

@@ -353,7 +353,7 @@ class TpDropdown extends BaseElement {
       this.dispatchEvent(new CustomEvent('is-open-changed', { detail: this.isOpen, bubbles: true, composed: true }));
     }
 
-    return true;
+    return super.shouldUpdate(changes);
   }
 
   updated(changes) {
@@ -802,7 +802,7 @@ class TpDropdown extends BaseElement {
     if (Array.isArray(this.items)) {
       for (var i = 0, li = this.items.length; i < li; ++i) {
         if (String(this.items[i].label).toLowerCase() === String(label)) {
-          this.value = this.items[i].id;
+          this.value = this.items[i].value;
           return true;
         }
       }