[whatwg/fetch] Clarify that request.origin cannot be "client" at certain points (PR #1776)

Previously, the code implicitly guaranteed that a requests origin could not be "client" after Step 10 of
https://fetch.spec.whatwg.org/#concept-fetch.

Other algorithms then simply relied on this fact. This is confusing. Instead, we now explicitly assert that the origin is not client whereever we need it.

This closes #1773

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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/1776

-- Commit Summary --

  * Clarify that request.origin cannot be "client" at certain points

-- File Changes --

    M fetch.bs (10)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1776.patch
https://github.com/whatwg/fetch/pull/1776.diff

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