Error: next() called multiple times
Hello,

when defined multiple http status codes in `output` error is thrown.
1. when only "400-599" is defined then ot works
2. when "400-599" and 200 are defined the error is thrown.

It seams that it has issues wtih multiple status codes.

```
const joiSchema = {
  body: {
    email: Joi.string()
      .lowercase()
      .email()
      .max(45)
      .required(),
    password: Joi.string()
      .max(10)
      .required()
  },
  output: {
    200: {
      body: {
        jwtToken: Joi.string()
          .alphanum()
          .min(3)
          .max(200)
          .required()
      }
    },
    "400-599": {
      body: {
        error: Joi.object()
      }
    }
  },
  type: "json",
  continueOnError: true
};
```

Error: next() called multiple times
      at dispatch (/home/kresimir/Development/zimmerfrei/zimmerfrei-http-server/node_modules/koa-router/node_modules/koa-compose/index.js:38:45)
      at next (/home/kresimir/Development/zimmerfrei/zimmerfrei-http-server/node_modules/koa-router/node_modules/koa-compose/index.js:45:18)
      at errorHandler (/home/kresimir/Development/zimmerfrei/zimmerfrei-http-server/node_modules/koa-joi-router/joi-router.js:285:22)
      at process._tickCallback (internal/process/next_tick.js:68:7)
