allow unsetting mixin with setAttribute('mixin', '') and removeAttribute

Author: processprocess

src/core/a-entity.js

@@ -567,6 +567,11 @@ var proto = Object.create(ANode.prototype, {
         return;
       }
 
+      if (attr === 'mixin') {
+        this.updateMixins();
+        return;
+      }
+
       HTMLElement.prototype.removeAttribute.call(this, attr);
     }
   },

src/core/a-node.js

@@ -131,6 +131,16 @@ module.exports = registerElement('a-node', {
       value: function (newMixins, oldMixins) {
         var newMixinsIds = newMixins ? newMixins.trim().split(/\s+/) : [];
         var oldMixinsIds = oldMixins ? oldMixins.trim().split(/\s+/) : [];
+
+        if (!newMixins) {
+          var mixinEls = this.mixinEls;
+          for (var i = 0; i < mixinEls.length; i++) {
+            this.unregisterMixin(mixinEls[i].id);
+          }
+          HTMLElement.prototype.removeAttribute.call(this, 'mixin');
+          return;
+        }
+
         // To determine what listeners will be removed
         var diff = oldMixinsIds.filter(function (i) { return newMixinsIds.indexOf(i) < 0; });
         this.mixinEls = [];