Method add

src/services/msgService.js:31–37

* 新增 msg * @param {Object} msgBody { title:{String}, content:{String} } * @return {Promise}

(msgBody)

   * @return {Promise}
   */
  add (msgBody) {
    return xhr({
      method: 'post',
      url: '/msg',
      body: msgBody
    })
  }

  /**
   * 修改 msg