[fix] ctx.request.origin ignores proxy flag (X-Forwarded-Proto)
## Describe the bug
When using the proxy flag set to true, ctx.request.origin ignores it, returning http:// instead of https:// (taken from X-Forwarded-Proto, available in ctx.protocol)

**Node.js version:** v16.14.0

**OS version:** Debian 10

**Description:** With koa's proxy flag to true, ctx.request.origin returns wrong protocol because it doesn't match with the X-Forwarded-Proto header (available in ctx.protocol).

## Actual behavior [app.proxy=true]

`http://mywebsite.com
`
## Expected behavior [app.proxy=true]

`https://mywebsite.com
`
## Code to reproduce
```
const Koa = require('koa');
const app = new Koa({ proxy: true });
app.use(ctx => {
  ctx.body = 'Hello Koa, origin:' + ctx.request.origin + '; protocol=' + ctx.protocol;
});

app.listen(3000);
```

I didn't submit a pull request since this is a very simple bug and it requires a reverse proxy setup (like nginx) to reproduce

## Checklist

- [X] I have searched through GitHub issues for similar issues.
- [X] I have completely read through the README and documentation.
- [X] I have tested my code with the latest version of Node.js and this package and confirmed it is still not working.
