fix detail_with_fields bug

if we assign fields attribute in admin object and someone just has view permission only to view details view
he/she will get a page with a updating form in the bottom

![error](https://f.cloud.github.com/assets/1411482/1510403/d31b62a0-4a7f-11e3-93aa-b36951e4b136.png)

if we use an account with change permission, it is ok
![normal](https://f.cloud.github.com/assets/1411482/1510410/098c24be-4a80-11e3-9cb3-122d6e960f44.png)


it is because we don't handle this situation in DetailAdminView when get_form_model and get_form_layout
now i fixed it trying my best
![fixed](https://f.cloud.github.com/assets/1411482/1510412/1475feae-4a80-11e3-9147-fab486c41754.png)