egg-http-proxy无法代理https请求
## What happens?
egg-http-proxy无法处理目标网站为https协议的网站

## 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.按照官方教程进行配置
2.await this.ctx.proxyRequest(xxxxxx)

**Expected behavior**
查看了一下eggjs/egg-http-proxy的源码，发现没有对协议进行处理

## Context
- **Node Version**:
- **Egg Version**:
- **Plugin Name**:
- **Plugin Version**:
- **Platform**:
