I think we should restrict `SLSwitch` to match _only_ instances of `UISwitch`:

(in `SLSwitch.m`):
```objc
- (BOOL)matchesObject:(NSObject *)object {
    return ([super matchesObject:object] && [object isKindOfClass:[UISwitch class]]);
}
```

There's no guarantee that `value` will be boolean, otherwise. Also, this best preserves parity with UIAutomation, where `UIASwitch` matches instances of `UISwitch`.