Assert that a request was made via a proxy server

I'm looking to write a test that ensures a http request happened, via a certain proxy server.

I can't seem to see any way to do that. Is it something you'd consider adding?

```ruby
stub_request(:post, "www.example.com").
  with(body: /world$/, headers: {"Content-Type" => /image\/.+/}, proxy: "http://myproxy.com").
  to_return(body: "abc")
```

perhaps?