fix(ref): allow ref key to be zero (vuejs#7676)

prevents missing elements when :ref value is "0"

fix vuejs#7669

Author: t0mburton

Diff for: src/core/vdom/modules/ref.js

@@ -1,6 +1,6 @@
 /* @flow */
 
-import { remove } from 'shared/util'
+import { remove, isDef } from 'shared/util'
 
 export default {
   create (_: any, vnode: VNodeWithData) {
@@ -19,7 +19,7 @@ export default {
 
 export function registerRef (vnode: VNodeWithData, isRemoval: ?boolean) {
   const key = vnode.data.ref
-  if (!key) return
+  if (!isDef(key)) return
 
   const vm = vnode.context
   const ref = vnode.componentInstance || vnode.elm

Diff for: test/unit/features/ref.spec.js

@@ -9,6 +9,10 @@ describe('ref', () => {
     test2: {
       id: 'test2',
       template: '<div>test2</div>'
+    },
+    test3: {
+      id: 'test3',
+      template: '<div>test3</div>'
     }
   }
 
@@ -20,6 +24,7 @@ describe('ref', () => {
       template: `<div>
         <test ref="foo"></test>
         <test2 :ref="value"></test2>
+        <test3 :ref="0"></test3>
       </div>`,
       components
     })
@@ -28,6 +33,8 @@ describe('ref', () => {
     expect(vm.$refs.foo.$options.id).toBe('test')
     expect(vm.$refs.bar).toBeTruthy()
     expect(vm.$refs.bar.$options.id).toBe('test2')
+    expect(vm.$refs['0']).toBeTruthy()
+    expect(vm.$refs['0'].$options.id).toBe('test3')
   })
 
   it('should dynamically update refs', done => {