fix optional parameters on AngularJS 1.4.9, fixed #31 and #34

Reference :
https://docs.angularjs.org/error/$compile/nonassign
https://docs.angularjs.org/api/ng/service/$compile#directive-definition-object

Author: dnauck

CHANGELOG.md

@@ -1,5 +1,6 @@
 ### 2.1.0 - unreleased
 * add support for suggested values for a search parameter with typeahead support, fixed #11
+* fix optional parameters on AngularJS 1.4.9, fixed #31 and #34
 
 ### 2.0.1 - 14 December 2015
 * enter and leave edit mode methods and events was not called every time due ng-if directive, fixed #17

src/angular-advanced-searchbox.js

@@ -18,9 +18,9 @@ angular.module('angular-advanced-searchbox', [])
                 model: '=ngModel',
                 parameters: '=',
                 parametersLabel: '@',
-                parametersDisplayLimit: '=',
+                parametersDisplayLimit: '=?',
                 placeholder: '@',
-                searchThrottleTime: '='
+                searchThrottleTime: '=?'
             },
             replace: true,
             templateUrl: function(element, attr) {