For which 1xx responses should Set-Cookie work?

[annevk]

Fetch currently requires Set-Cookie to be processed for a response whose status is 101 (though only when request's mode is "websocket") and ignored when response status is any other 1xx (or 101 and request's mode is not "websocket").

Is that the behavior we want?

Relates to httpwg/http-extensions#2084.

Credit Guoye Zhang if we end up making a change to the specification.

[mnot]

Current cookie spec says:

User agents MAY ignore Set-Cookie header fields contained in responses with 100-level status codes or based on its cookie policy (see Section 7.2).