Fix issubset implementation

Current implementation is not valid as it will return false when the element is
not present on one of the list, but it should return false only if the element
is not on both lists.

issue5669
review28391002

src/model.js

@@ -1126,7 +1126,7 @@
             if (!jQuery.isEmptyObject(fields)) {
                 var result = true;
                 fields.forEach(function(field) {
-                    if (!(field in this._loaded) | !(field in this._changed)) {
+                    if (!(field in this._loaded) && !(field in this._changed)) {
                         result = false;
                     }
                 }.bind(this));