Fix for tabs leaving content behind

sometimes, dynamically created tabs will fail to update the `active` attribute, so inactive tab content is not hidden. explicitly unset the attribute on the old tab and set it on the new tab to prevent this from happening.

Author: shuwen

src/mm-tabs/mm-tabs.js

@@ -44,9 +44,10 @@
 		},
 
 		_selectedIndexChanged: function(newIndex, oldIndex) {
-			if(newIndex !== oldIndex) {
+			if(this._tabs && newIndex !== oldIndex) {
 				this.set('_tabs.'+oldIndex+'.active', false);
 				this.set('_tabs.'+newIndex+'.active', true);
+				this.attributeFollows('active', this._tabs[newIndex], this._tabs[oldIndex])
 			}
 		},
 
@@ -89,4 +90,4 @@
 
 	});
 
-})(window.Strand = window.Strand || {});
+})(window.Strand = window.Strand || {});