code: def fetch(self):
          
          resp = self.r_session.get(self.document_url)
          resp.raise_for_status()
          self.clear()
          self.update(response_to_json_dict(resp))
docs: Retrieves the content of the current security document from the remote
          database and populates the locally cached SecurityDocument object with
          that content.  A call to fetch will overwrite any dictionary content
          currently in the locally cached SecurityDocument object.