[dev] Handle response headers

[EverlastingBugstopper]

Fixes #872

This is now a pretty usable dev server! There's still a bunch of stuff to do but this is what I'd considered an MVP.

---

worker code:

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request))
})

async function handleRequest(request) {
  return new Response("hello world", { headers: {"x-test": "header"}})
}

---

Proxy and curl running in parallel

$ curl --head localhost:8000
HTTP/1.1 200 OK
content-length: 11
content-type: text/plain;charset=UTF-8
x-test: header
date: Wed, 20 Nov 2019 17:33:25 GMT

$ wrangler dev
Listening on http://localhost:8000
[2019-11-20 11:33:25] "HEAD example.com/ HTTP/1.1"

[ashleymichal]

i would like to see a couple of comments again, but i won't block on it.

[ashleymichal]

i think this closure deserves to either be a well named function in its own right, or to have a comment above.

[EverlastingBugstopper]

i don't think i can do that since hyper doesn't re-export the type MakeServiceFn from what I can tell. i've simplified the closure as much as i can by pulling some if it out into a separate function though

[gabbifish]

lil nit, otherwise lgtm. I'd love for it to be addressed though--this is substantial enough that I give the previous PR with the same feedback a request change :)

[EverlastingBugstopper]

@gabbifish @ashleymichal i think i've addressed concerns here :)