Fix signature for destroy callback function.

+ Clear callback model instead of persistent model.
Add missing return statement for Backbone sync.
Fix duplicate storage of the user name in the user model.

+ API response for sign-in: { auth_token: 123, user: { name: Alf, ...  } }
+ Also see commit 3df9710cee09db917e9fa9eada7ff3857dfd53a1.
Ensure Backbone sync uses "create" method on sign-in.

+ Default `id` caused "update" instead of "create"
  because `user.isNew()` returned false.
Explicitly expect "create" method in Backbone sync for sign-in.
Extended session specs to test response body content.