[Bug]: Websockets/SSEs do not work on custom domains if proxied by Cloudflare

### Problem description

I haven't been able to reproduce the issue outside of Deploy, but Websockets and SSEs fail to work on a custom domain when proxied by Cloudflare. This means that websites built with Fresh would outright not work at all.

### Steps to reproduce

1.) Clone this [sample repo](https://github.com/sr229/kuru-kuru) or any sample repo with SSEs.
2.) Deploy it on Deno Deploy
3.) Attach a custom domain to the deployment, it's very important that the domain must be backed by Cloudflare
4.) Add your DNS entries, and leave it proxied.
5.) Visit the site and test against the deno.dev version.

### Expected behavior

SSEs and Websockets should be able to work in Cloudflare, but it somehow doesn't

### Environment

- Deno: Version 1.37.2
- OS: Windows 10 on Edge 118.

### Possible solution

Most likely you'll have to be able to allow Cloudflare proxy to access the site properly and/or coordinate with Cloudflare for a fix.

### Additional context

_No response_