[fetch] HTTPAUTH reference (#136)

RFC 2617 is now obsolete and has been replaced by four documents (see https://lists.w3.org/Archives/Public/ietf-http-wg/2015OctDec/0011.html for details).

In the context of FETCH, updating the reference to refer to RFC 7235 is probably sufficient (unless something specific to Basic or Digest is needed).

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/136