[Javascript] Add initialization of default vars to constructor (#19692)

* Add initialization of default vars to constructor

* Update samples

samples/client/petstore/javascript-es6/src/model/Animal.js

@@ -36,6 +36,7 @@ class Animal {
      */
     static initialize(obj, className) { 
         obj['className'] = className;
+        obj['color'] = 'red';
     }
 
     /**

samples/client/petstore/javascript-es6/src/model/Foo.js

@@ -34,6 +34,7 @@ class Foo {
      * Only for internal use.
      */
     static initialize(obj) { 
+        obj['bar'] = 'bar';
     }
 
     /**

samples/client/petstore/javascript-es6/src/model/Order.js

@@ -34,6 +34,7 @@ class Order {
      * Only for internal use.
      */
     static initialize(obj) { 
+        obj['complete'] = false;
     }
 
     /**