Strange behavior for passport.authenticate() function
Hi there!

I'm trying koa-passport and discovered a strange behavior on passport.authenticate() function call.
This function does ```return next()``` and this breaks my actions sequence (see details below).
If I change ```return next()``` in koa-passport lib to ```return```, everything works just fine (at least as I expect it to be).

Can anyone advice on this and help me to clarify if there is a bug or I'm just doing something wrong?

My example application code that makes output located at:
https://gist.github.com/Brozish/69e8147f323bbb044f3efd21e5d0df2f

create file version.js and copy example application code
```
yarn init
yarn add koa koa-router koa-bodyparser koa-passport passport-jwt mongoose winston
node version.js
```

Query application with curl or similar tool to see output:

```bash
curl -X GET \
  http://localhost:3000/users \
  -H 'Authorization: Bearer eyJhbGciOiJIUzUxMiIsInR5cCI6IkpXVCJ9.eyJpZCI6IjViZmQwNTYwYWZmZjcxMjBmOTBkZTY4NSIsImVtYWlsIjoiaXNzdWVAaXNzdWUuaXNzdWUiLCJpYXQiOjE1NDMzMDg2NTcsImV4cCI6MTU3NDg0NDY1N30.RuyTMiiZRS6j8XhWPzPSKL4jacn2aRnaWLQQ6K5z9F7aWQgBoehl-1eXddRy_QfSxRF4cILPUhSWa1PnK78ocg' \
  -H 'cache-control: no-cache'
```

My application console output if koa-passport does ```return next()``` (wrong actions sequence):

- startAuth
- beforeKoaPassport
- jwtStrategy
- startGetUsers
- endGetUsers
- afterKoaPassport
- endAuth

My application console output if koa-passport does ```return``` (everything is OK):

- startAuth
- beforeKoaPassport
- jwtStrategy
- afterKoaPassport
- endAuth
- startGetUsers
- endGetUsers

Koa-passport lib code that breaks my app can be found at:
https://github.com/rkusa/koa-passport/commit/116fa48e2ca0ee6ac362f8b91e44e8e52a4ef30a

git blame ./lib/framework/koa.js --date=short -L 14
116fa48e (rkusa        2015-11-20 149)         return next()

My full application code:
https://bitbucket.org/Brozish/node.js/src/master/

Any help on this subject would be appreciated. If this is a bug, I would be happy to make a pull request or being mentioned in commit with a fix.