Model generation without attribute type

`ember g model user username password`

would yield
```
export default DS.Model.extend({
  username: DS.attr(),
  password: DS.attr()
});
```
"If you don't specify the type of the attribute, it will be whatever was provided by the server"
http://emberjs.com/guides/models/defining-models/
Can you add the PR number as a link here?