NSDictionary setValue: forKey:的坑

setValue: forKey是NSMutableDictionary的一个扩展方法，当传入的value为nil时为调用removeObjectForKey:方法，对于NSDictionary来说没有removeObjectForKey:
方法，所以会报错 setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key xxx