code: def _add_version_to_request(self, url, headers, version):
          
          if self._has_capability(SERVER_REQUIRES_VERSION_HEADER):
              new_headers = headers.copy()
              new_headers['Last-Modified'] = email.utils.formatdate(version)
              return url, new_headers
          else:
              url_params = {
                  'last_modified': email.utils.formatdate(version)
              }
              new_url = url + "?" + urlencode(url_params)
              return new_url, headers
docs: Adds version to either url or headers, depending on protocol.