localstorage: Correctly check whether value is set before setting default

The old condition always overwrote boolean values that were set to false
with the default m( Check for undefined instead.

js/localstorage.js

@@ -98,7 +98,7 @@ ls.factory("$store",function($parse){
              */
             bind: function ($scope, key, def) {
                 def = def || '';
-                if (!publicMethods.get(key)) {
+                if (publicMethods.get(key) === undefined) {
                     publicMethods.set(key, def);
                 }
                 $parse(key).assign($scope, publicMethods.get(key));