Use string key with Hash#fetch

Not sure if this is working on Ruby 1.9 but I'm on 2.1.1p76 and symbol keys aren't working with fetch.

### Before changes

![screen shot 2014-04-14 at 10 07 57 pm](https://cloud.githubusercontent.com/assets/898172/2703015/4e13f28c-c443-11e3-9211-483aee613d69.png)

### After changes

![screen shot 2014-04-14 at 10 08 17 pm](https://cloud.githubusercontent.com/assets/898172/2703017/539adbc6-c443-11e3-9e0d-945e9fea6c5e.png)