'next is not a function' in middleware
Hello all, I have an issue when trying to add Sequelize DB to state of ctx:

add-db-to-state.js
```js
const db = require('../libs/db');

const logger = require('../helpers/get-logger')(__filename);

const addPostgreSQLToState = () => async (ctx, next) => {
  logger.debug('addDBToState');

  const connection = await db.getConnection();

  ctx.state.db = connection;

  await next();
};

module.exports = addPostgreSQLToState;
```

app.js
```js
const addDBToState = require('./middlewares/add-db-to-state');
...
app.use(addDBToState());
...
```

Log I get:

```
[2018-07-10T17:16:22.582Z] [error] /src/middlewares/errors.js - errors - path=/auth/user/, state=[Circular], message=next is not a function, stack=TypeError: next is not a function
    at initialize (/node_modules/passport/lib/middleware/initialize.js:53:5)
    at dispatch (/node_modules/koa-compose/index.js:42:32)
    at /home/.../src/middlewares/add-db-to-state.js:12:9
    at process._tickCallback (internal/process/next_tick.js:68:7) 
[2018-07-10T17:16:22.583Z] [info] /src/middlewares/requests.js - response - status=500, time=13ms, body={ username: qoqoqoq13311, email: qweqwe@aa.aa11, fullName: qweqweqweqweq, passwordConfirmation: qweqqweq } 

```