extjs_associations does not count with user-defined foreign_key

Something like this will break:

```
belongs_to :created_by, :class_name => 'User', :foreign_key => :created_by_user_id
```

I fixed it by changing code on lib/model/active_record.rb:63 to

```
:foreign_key => assn.options[:foreign_key] || assn.association_foreign_key
```
