Modifying the es5 and es6 templates for javascript to handle default values (#6649)

for required fields better. This change ensures the required field isn't
overridden with undefined when the object is constructed - especially through
a chain of constructFromObject calls..

samples/client/petstore/javascript-promise-es6/src/model/Category.js

@@ -35,7 +35,7 @@ class Category {
      * Only for internal use.
      */
     static initialize(obj, name) { 
-        obj['name'] = name;
+        obj['name'] = name || 'default-name';
     }
 
     /**