code: func (m *Manager) getRefreshToken(token string) (rt *refreshToken, err error) {
  	rt = &refreshToken{}
  
  	err = m.getRefreshTokenCollection().Find(bson.M{"refreshtoken": token}).One(rt)
  	if err == mgo.ErrNotFound {
  		rt = nil
  		err = nil
  	}
  	if err != nil {
  		rt = nil
  	}
  
  	return
  }
docs: //getRefreshToken gets an refresh token by it's refresh token string
  // If the token is not found or is expired, nil is returned