Create the user agent and return it as a string.

def _create_user_agent(self):
        """ Create the user agent and return it as a string. """
        user_agent = '{}/{} {}'.format(pyspacegdn.__title__,
                                       pyspacegdn.__version__,
                                       default_user_agent())
        if self.client_name:
            user_agent = '{}/{} {}'.format(self.client_name,
                                           self.client_version, user_agent)
        return user_agent