observer conversion enumeration should not include prototype properties

src/core/observer/index.js

@@ -58,9 +58,9 @@ export class Observer {
    * value type is Object.
    */
   walk (obj: Object) {
-    const val = this.value
-    for (const key in obj) {
-      defineReactive(val, key, obj[key])
+    const keys = Object.keys(obj)
+    for (let i = 0; i < keys.length; i++) {
+      defineReactive(obj, keys[i], obj[keys[i]])
     }
   }