[whatwg/fetch] Correct some header list append invocations (#1057)

As well as a typo.

Closes #894 and closes #895.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/1057

-- Commit Summary --

  * Correct some header list append invocations

-- File Changes --

    M fetch.bs (11)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1057.patch
https://github.com/whatwg/fetch/pull/1057.diff

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