why do you use 'koa-json' in koa 2 version ?
I find koa-json does not support koa 2 version, [see link](https://github.com/koajs/koa/wiki#json-and-jsonp-responses)

and I find 'koa-bodyparser' also can support json , [A body parser for koa, base on co-body. support json, form and text type body.](https://github.com/koajs/bodyparser)

so why do you still use it in koa 2 version not only use koa-bodyparser ? thanks

