also make _ and $ prefixed properties reactive, but not proxying them

src/observer/index.js

@@ -82,9 +82,7 @@ Observer.prototype.walk = function (obj) {
   while (i--) {
     key = keys[i]
     prefix = key.charCodeAt(0)
-    if (prefix !== 0x24 && prefix !== 0x5F) { // skip $ or _
-      this.convert(key, obj[key])
-    }
+    this.convert(key, obj[key])
   }
 }