[fetch] Provide opt-out for HTTP proxy authentication? (#42)

Currently all fetches will tee the request body due to these three reasons:

1. Redirects
2. HTTP authentication
3. HTTP proxy authentication

1 can be disabled through `redirect: "error"`. 2 is already disabled for `fetch()` (handling it automatically and prompting the user that is). 3 cannot be disabled at the moment.

The question is whether it would be acceptable to provide an option to error on 3 as well and thereby opening up the ability to make a client-side decision not to tee request's body.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/42