ctx.accept seems to be a unused property
https://github.com/koajs/koa/blob/13086d2fcd5559296f473b4c7bbd56021f9cb373/lib/application.js#L175
`request.accept` could only be accessed indirectly through `request.accepts()`. But there is no method or getter/setter using the `ctx.accept` and it is not declared as a public API.