NSJSONSerialization 转换时 类型注意

进行JSON转化的时候，需要满足一下的要求。

An object that may be converted to JSON must have the following properties:

• The top level object is an NSArray or NSDictionary.
• All objects are instances of NSString,NSNumber,NSArray,NSDictionary,or NSNull.
• All dictionary keys are instances of NSString.
• Numbers are not NaN or infinity.

也就是说，

1. 最上面层，必须是 NSArray 或 NSDictionary
2. 所有的value 必须是 NSString,NSNubmer,NSDictionar 或 NSNull.
3. 所有的key 必须是 NSString
4. 并且Numbers 不能是 NaN 或 无穷大。