Koa v2 Middleware

Hi,

Are you going to migrate this module to [koa v2 middleware signature](https://github.com/koajs/koa/blob/HEAD/docs/migration.md)?

Start by reading the linked Koa v2 migration guide, then inspect the repository's user-auth middleware implementation to identify its current middleware signature. Done means the module follows the Koa v2 signature and remains usable as Koa middleware; the issue does not name tests or a specific file to run.

Written by the indexing model from the issue text.