fix class updates if new vnode doesnt contain any class data

Author: yyx990803

src/platforms/web/runtime/modules/class.js

@@ -5,7 +5,9 @@ import { genClassForVnode, concat, stringifyClass } from 'web/util/index'
 function updateClass (oldVnode: any, vnode: any) {
   const el = vnode.elm
   const data: VNodeData = vnode.data
-  if (!data.staticClass && !data.class) {
+  const oldData: VNodeData = oldVnode.data
+  if (!data.staticClass && !data.class &&
+      (!oldData || (!oldData.staticClass && !oldData.class))) {
     return
   }
 

test/unit/modules/vdom/modules/class.spec.js

@@ -92,4 +92,16 @@ describe('vdom class module', () => {
     expect(elm).not.toHaveClass('class2')
     expect(elm).not.toHaveClass('class3')
   })
+
+  it('should remove class for new nodes without class data', () => {
+    const vnode1 = new VNode('p', {
+      class: { class1: true, class2: false, class3: true }
+    })
+    const vnode2 = new VNode('p', {})
+    let elm = patch(null, vnode1)
+    elm = patch(vnode1, vnode2)
+    expect(elm).not.toHaveClass('class1')
+    expect(elm).not.toHaveClass('class2')
+    expect(elm).not.toHaveClass('class3')
+  })
 })