Update the etag from an API response.

    Args:
      response: HTTP response with a header field.

    Returns:
      bool, True if the etag in the response header updated.

def _UpdateEtag(self, response):
    """Update the etag from an API response.

    Args:
      response: HTTP response with a header field.

    Returns:
      bool, True if the etag in the response header updated.
    """
    etag = response.headers.get('etag', self.etag)
    etag_updated = self.etag != etag
    self.etag = etag
    return etag_updated