Add updateItem test for field Boolean.

I'm playing around with [boolean.prototype.updateItem(),](https://github.com/JedWatson/keystone/blob/master/lib/fieldTypes/boolean.js#L53)  so I figured having a test would be a good idea before I start tweaking.

I suspect this method is the source of my problems mentioned in #201.

PS: I'm not sure what the plan is for structuring the tests. I wanted to initialize the 'Test' list in a beforeEach() block for isolation but that broke the other tests. I used before() instead. Let me know if I'm doing this right. :+1: