UIActionSheet tests redux

This restores and tweaks the `UIActionSheet` tests removed in #705.

The quick fix is testing by invoking the delegate method directly. This is how `UIAlertView` is also [tested](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/b3a408c2d437a24d98b32256845cd1bedaea1bf3/ReactiveCocoaFramework/ReactiveCocoaTests/UIAlertViewRACSupportSpec.m#L29). This isn't awesome, but better than nothing.

Do we make any distinction between unit testing and integration testing?