code: def refresh(self):
          
          super().refresh()
          self._html = None
          self._upvote_num = None
          self._content = None
          self._collect_num = None
          self._comment_num = None
docs: 刷新 Answer object 的属性. 
          例如赞同数增加了, 先调用 ``refresh()`` 
          再访问 upvote_num属性, 可获得更新后的赞同数.
          
          :return: None