Fix an issue where the beSupersetOf matcher is broken with NSCFDictionaries.

The `beSupersetOf` matcher breaks with dictionaries generated by some Foundation API methods (e.g. `NSJSONSerialization#JSONObjectWithData`). This is because the `isKindOfClass` method fails to identify a NSCFDictionary as a kind of NSDictionary.

This PR fixes this issue by assuming the classes match if they are both dictionaries.