Modify httpRequest’s header list per HTTP

[ronag]

We don't know what this means in the spec:

18. Modify httpRequest’s header list per HTTP. Do not append a given header if httpRequest’s header list contains that header’s name.

Refs: https://fetch.spec.whatwg.org/#http-network-or-cache-fetch
Refs: whatwg/fetch#1285

[ronag]

I believe we do this now... it's unclear.