Fixed _resetCacheForKey

Author: killme2008

lib/object.js

@@ -460,11 +460,13 @@
       if (_.isObject(value) &&
           !(value instanceof AV.Object) &&
           !(value instanceof AV.File)) {
+
         value = value.toJSON ? value.toJSON() : value;
         var json = JSON.stringify(value);
         if (this._hashedJSON[key] !== json) {
+          var wasSet = !! this._hashedJSON[key];
           this._hashedJSON[key] = json;
-          return true;
+          return wasSet;
         }
       }
       return false;