$add should notify regardless of whether the ob has associated vms (fix vuejs#873)

Author: yyx990803

src/observer/object.js

@@ -21,15 +21,14 @@ _.define(
       return
     }
     ob.convert(key, val)
+    ob.notify()
     if (ob.vms) {
       var i = ob.vms.length
       while (i--) {
         var vm = ob.vms[i]
         vm._proxy(key)
         vm._digest()
       }
-    } else {
-      ob.notify()
     }
   }
 )
@@ -70,15 +69,14 @@ _.define(
     if (!ob || _.isReserved(key)) {
       return
     }
+    ob.notify()
     if (ob.vms) {
       var i = ob.vms.length
       while (i--) {
         var vm = ob.vms[i]
         vm._unproxy(key)
         vm._digest()
       }
-    } else {
-      ob.notify()
     }
   }
 )