message: fix NavBar test cases
diff: exports[`renders demo correctly 1`] = `
  <div>
  <div>
  - <div class="van-hairline--bottom van-nav-bar" style="z-index:1;">
  + <div class="van-nav-bar van-hairline--bottom" style="z-index:1;">
  <div class="van-nav-bar__left">
  <i class="van-icon van-icon-arrow van-nav-bar__arrow" style="color:undefined;font-size:undefined;">
  <!---->
  @@ -13,7 +13,7 @@ exports[`renders demo correctly 1`] = `
  </div>
  </div>
  <div>
  - <div class="van-hairline--bottom van-nav-bar" style="z-index:1;">
  + <div class="van-nav-bar van-hairline--bottom" style="z-index:1;">
  <div class="van-nav-bar__left">
  <i class="van-icon van-icon-arrow van-nav-bar__arrow" style="color:undefined;font-size:undefined;">
  <!---->
  