[Mime] Re-allow addIdHeader to be used for 'In-Reply-To' and 'References' headers #47225
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#44732 causes messages using
addIdHeader('In-Reply-To', ...)
oraddIdHeader('References', ...)
to fail the headers check (see #44732 (comment) ), requiring users to manually format the header value themselves.This PR re-allows the previous behaviour of
addIdHeader
while keeping the new unstructured behaviour in-place as the default.(I'm not sure if this counts as a new feature or a bug fix since it "reintroduces" a previous feature)