return fetch body directly

[syumai]

What

• add jsutil.RawJSWriter and use it in jshttp.ResponseWriter.
  • This passes raw JS fetch response body as Cloudflare Worker's response body without using Go's io.Reader.

Motivation

• To support streaming. see: Can fetch pkg support streaming? #85.

How to use this feature?

• Please use the cloudflare/fetch package and io.Copy to implement the proxy.
• See _examples/basic-auth-proxy/main.go.