使用[NSJSONSerialization dataWithJSONObject：]转换为NSData时保持NSDictionary键的顺序

I have the following situation:

我有以下情况：

NSDictionary *params = @{
    @"Checkout" : @{
        @"conditions" : @{@"Checkout.user_id" : @1},
        @"order" : @{@"Checkout.id" : @"DESC"}
    },
    @"PaymentState" : @[],
    @"Offer" : @[]
};
NSD