fix #871 - calling set with 'attributes' property

backbone.js

@@ -220,7 +220,7 @@
       // Extract attributes and options.
       options || (options = {});
       if (!attrs) return this;
-      if (attrs.attributes) attrs = attrs.attributes;
+      if (attrs instanceof Backbone.Model) attrs = attrs.attributes;
       if (options.unset) for (var attr in attrs) attrs[attr] = void 0;
       var now = this.attributes, escaped = this._escapedAttributes;
 

test/model.js

@@ -531,4 +531,10 @@ $(document).ready(function() {
     model.set({a: 'a', b: 'b', c: 'c'});
   });
 
+  test("#871, set with attributes property", function() {
+    var model = new Backbone.Model();
+    model.set({attributes: true});
+    ok(model.has('attributes'));
+  });
+
 });