Use this.disabled to consider the case when parent selectionList is disabled?

Author: sllethe

src/lib/list/list-option.ts

@@ -142,7 +142,7 @@ export class MdListOption implements AfterContentInit, OnDestroy, Focusable {
   }
 
   _handleClick() {
-    if (!this._disabled) {
+    if (!this.disabled) {
       this.toggle();
     }
   }