Issue title: How do I add a new attribute to an existing object of Keystonejs list ?
 Issue body: ``` javascript
keystone.list(‘MyModel').model.findOne( { _id: searchId } , function (err, mymodel) {
      mymodel.oldAttribute = “Changed Old VAL";
      mymodel.newAttribute = “Entered New VAL";
      mymodel.save(function (err) {
          if(err) { throw(err); }
      });
 });
```

But when I check after the update I get only this,

``````
{ _id: 55e17723b4856b85b1cf5f0a,  updatedAt: Sat Aug 29 2015 15:40:18 GMT+0530 (IST),key: '55e17723b4856b85b1cf5f0a',  __v: 0,  oldAttribute: 'Changed Old VAL',  createdAt: Sat Aug 29 2015 14:40:59 GMT+0530 (IST) }```
``````

 Comments: 
 Comment 0: Have you defined newAttribute in the MyModel list?
 Comment 1: Got it !!! Thanks
{'source': 'GithubIssues', 'repo_name_with_owner': 'keystonejs/keystone-classic', 'issue_id': 103843711, 'repo_stars': 14712.0}