Get "app" instance in middleware
## What happens?
Unable to get the app instance in middlewares

## Mini Showcase Repository(REQUIRED)
> Provide a mini GitHub repository which can reproduce the issue.
> Use `npm init egg --type=simple bug` then upload to your GitHub

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## How To Reproduce

**Steps to reproduce the behavior:**
1. I am unable to put the authentication code inside middleware instead of controller. the app.passport is inaccessible in middleware.

**Expected behavior**
1. Able to access app.passport directly from middleware and handle the authentication

## Context
- **Node Version**: 10.19.0
- **Egg Version**: 2.15.1
- **Plugin Name**: egg-passport
- **Plugin Version**: 2.1.1
- **Platform**: Ubuntu
