Minor cleanup.

@singingwolfboy A couple of minor things I noticed while using the textbooks as an example for assets.

1) The _.result syntax isn't working. I'm not sure why, but the asserts always pass, no matter what you are testing for. Calling the methods directly seems simpler anyway.

2) Related to #1, the URL key was wrong for CMS.Collections.TextbookSet.

3) Calling destroy on a model removes it from the collection automatically.