Delete a cookie (by setting it to a blank value).

        The path and domain values must match that of the original cookie.

def delete_cookie(self, key, path='/', domain=None):
        """Delete a cookie (by setting it to a blank value).

        The path and domain values must match that of the original cookie.
        """
        self.set_cookie(key, value='', max_age=0, path=path, domain=domain,
                        expires=datetime.utcfromtimestamp(0))