[dev] Handles cf-ew-raw- headers

[EverlastingBugstopper]

Fixes #873

worker code returns stringified headers in the response body:

async function handleRequest(request) {
  let headers = new Map(request.headers)
  return new Response(mapToString(headers))
}

$ curl http://localhost:8000 -H "Hi: There"
{"accept":"*/*","hi":"There","host":"example.com","user-agent":"curl/7.64.1","x-forwarded-proto":"https"}

[gabbifish]

Small request!

[gabbifish]

Some nits as well as suggestions for moving variables around :)

[gabbifish]

do we need to clone this twice? (once in line 49, once here?) I feel like we can put variables like client, preview_id, and server_config in the arguments section move |args| and avoid unnecessary clones :)

[gabbifish]

Looks great!!