fix(LeanObject): test existence more accurately

Previously, the code checks whether the object has an truthy objectId,
to determine the existence of a LeanCloud object.
But if the object is constructed via `query.get("nonexist_objectId")`,
then it does have an objectId (`{"objectId": "nonexist_objectId"}`).
Therefore, we check whether the object has the `createdAt` attribute
instead.

Also update the comments of `is_new` and `is_existed`,
to clarify their intented usage.

Author: weakish

leancloud/object_.py

@@ -517,12 +517,21 @@ def is_new(self):
         """
         判断当前对象是否已经保存至服务器。
 
+        该方法为 SDK 内部使用（save 调用此方法 dispatch 保存操作为 REST API 的 POST 和 PUT 请求）。
+        查询对象是否在服务器上存在请使用 is_existed 方法。
+
+
         :rtype: bool
         """
         return False if self.id else True
 
     def is_existed(self):
-        return bool(self.id)
+        """
+        判断当前对象是否在服务器上已经存在。
+
+        :rtype: bool
+        """
+        return self.has("createdAt")
 
     def get_acl(self):
         """