[fetch] Provide opt-in for Expect = "100-continue" (#41)

We could avoid teeing the request body with this protocol feature: http://httpwg.github.io/specs/rfc7231.html#header.expect

We would need to decide how much of the protocol is implemented in Fetch and how much we leave up to developers.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/41