Local Stragegy error:  "TypeError: next is not a function"
Hi, awesome middleware!

I'm getting that error when i use local-strategy with this middleware.

The examples work perfectly but i get these errors when i use it by myself.
Here is my code.

``` js
//- In passport.js file
import passport from 'koa-passport';
import { Strategy as LocalStrategy } from 'passport-local';

//- Simplified Local strategy...
const localLogin = new LocalStrategy({}, (username, password, done) => {
    User.findOne({username, password}, done);
});

//...Then later
passport.use(localLogin);
```

``` js
//- In my app.js file

const app = new koa(); 

app.use(convert(body({
    multipart: true,
})));

app.use(authApi.middleware());
app.use(api.middleware());

app.use(convert(passport.initialize()));
app.use(convert(passport.session()));

export default app;
```
and in my routes i have ..

``` js
//- Routes file.

import Router from 'koa-rest-router';
import { signIn, signUp } from './../controller/authentication';
import passport from 'passport';

const requireAuth = passport.authenticate('jwt', { session: false });
const requireSignIn = passport.authenticate('local', { session: false });

router.post('/login', requireSignIn, signIn);
```

The `requireSignIn` is the one called first and `signIn` won't even get called when that error fires!

what's interesting is i've tried to replace `local` in
 ``` js
const requireSignIn = passport.authenticate('local', { session: false });
```

with 
``` js
const requireSignIn = passport.authenticate('whatever', { session: false });
```

and i get the very same error message!

It's like my local-strategy won't get recognized for some reason.

**Stack Trace**

``` bash
 TypeError: next is not a function
    at attempt (/home/user/Projects/koa/test-project1/node_modules/passport/lib/middleware/authenticate.js:173:32)
    at authenticate (/home/user/Projects/koa/test-project1/node_modules/passport/lib/middleware/authenticate.js:349:7)
    at dispatch (/home/user/Projects/koa/test-project1/node_modules/koa-compose/index.js:44:32)
    at /home/user/Projects/koa/test-project1/node_modules/koa-compose/index.js:36:12
    at /home/user/Projects/koa/test-project1/node_modules/koa-better-router/index.js:544:46
    at dispatch (/home/user/Projects/koa/test-project1/node_modules/koa-compose/index.js:44:32)
    at next (/home/user/Projects/koa/test-project1/node_modules/koa-compose/index.js:45:18)
    at createGenerator (/home/user/Projects/koa/test-project1/node_modules/koa-convert/index.js:24:16)
    at createGenerator.next (<anonymous>)
    at Function.parseBody (/home/user/Projects/koa/test-project1/node_modules/koa-better-body/utils.js:269:20)
    at parseBody.next (<anonymous>)
    at Object.plugin (/home/user/Projects/koa/test-project1/node_modules/koa-better-body/index.js:50:21)
    at plugin.next (<anonymous>)
    at onFulfilled (/home/user/Projects/koa/test-project1/node_modules/co/index.js:65:19)
    at process._tickCallback (internal/process/next_tick.js:103:7)

```
Thank You.