Re: [fetch] Only append Authorization if none is set (#198)

These tests are on the way into web-platform-tests:
https://github.com/w3c/web-platform-tests/issues/2503
https://github.com/w3c/web-platform-tests/issues/2502

..and one is yet to be written:
https://github.com/w3c/web-platform-tests/issues/2512

@wanderview if you can think of scenarios that are not yet covered by those tests and the ones you see if going to https://github.com/w3c/web-platform-tests/find/master and typing ```send-authentication-``` please let me know :)

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