[Nimble] Use switch-case statement in Contain Matcher

If we don't use `if let x = actual {}`, it causes a compile error like the screenshot.

![2014-06-20 15 27 27](https://cloud.githubusercontent.com/assets/909674/3337088/1a8f292e-f844-11e3-81b4-6101d299f2e0.png)