Refactor SessionsController test

This change is already merged, but I wanted to add some notes explaining some of the changes I made. Once my notes have been reviewed, this PR can be closed.
Thinking about it, it makes sense that you can call `expect(growlNotifications.add)` to create your expectation since when you create a spy it replaces the method it is spying on with the spy, but I've always set my spies to variables and then later referred to those variables when building expectations. I like your way better.