refactor(AVObject): simplify get Created/UpdatedAt (#568)
refactor(AVObject): simplify get Created/UpdatedAt (#568)

`this.get('createdAt')` itself evaluates to
`this.createdAt`,
thus `this.createdAt || this.get('createdAt')` is unnecessary.
The same applies to `getUpdatedAt`.
leancloud/javascript-sdk,leancloud/javascript-sdk,leancloud/javascript-sdk