observer: modify '__ob__' property checking

src/observer/index.js

@@ -47,7 +47,7 @@ Observer.create = function (value, vm) {
   }
   var ob
   if (
-    '__ob__' in value &&
+    Object.prototype.hasOwnProperty.call(value, '__ob__') &&
     value.__ob__ instanceof Observer
   ) {
     ob = value.__ob__