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.
Injecting the $injector is a good play here, in retrospect, possibly a better one than I went with. However, I took a different approach so I could demonstrate a special behavior of test injection.

When injecting into a test you can inject any injectable by it's usual name or with a leading and trailing underscore. This underscored format allows you to inject something without overriding variables with the same name in surrounding scopes. That's kind of a weird sentence/notion, so here's an example:

Above I declared variables for `currentUser`, `growlNotifications`, and `$httpBackend` in the scope of the `describe` block. If I injected any of the resources with those names into the `inject` block by their real names, I would get the requested resource, but I would no longer have access to the variables with the same name defined in the `describe` block.

This isn't necessarily a problem, but in this case I defined those variables in the scope of the `describe` block because I want to be able to inject them in a DRY and consistent fashion for all of the tests within the `describe` block.

So if I injected `$httpBackend` instead of `_$httpBackend_`, I would get back the expected resource, but I wouldn't be able to set the `$httpBackend` variable to make the resource available in all the tests. However, since the resource can be injected as `_$httpBackend_`, I can get the resource, do whatever I want with it, then assign it to the `describe` block scoped variable, thereby making my modified resource available to all the tests.

This is usually most handy when modifying a resource for the test, but can also be useful in reducing the need to wrap every test in a call to `inject`. For example, the tests below no longer need to each inject `$httpBackend` because it is taken care of during the injection in the call to `beforeEach`.