* Drop Origin & Accept from Access-Control-Allow-Headers value

This change drops the Origin and Accept header names from the value of the
Access-Control-Allow-Headers response header sent by Synapse. Per the CORS
protocol, it’s not necessary or useful to include those header names.

Details:

Per-spec at https://fetch.spec.whatwg.org/#forbidden-header-name, Origin
is a “forbidden header name” set by the browser and that frontend
JavaScript code is never allowed to set.

So the value of Access-Control-Allow-Headers isn’t relevant to Origin or
in general to other headers set by the browser itself — the browser
never ever consults the Access-Control-Allow-Headers value to confirm
that it’s OK for the request to include an Origin header.

And per-spec at https://fetch.spec.whatwg.org/#cors-safelisted-request-header,
Accept is a “CORS-safelisted request-header”, which means that browsers
allow requests to contain the Accept header regardless of whether the
Access-Control-Allow-Headers value contains "Accept".

So it’s unnecessary for the Access-Control-Allow-Headers to explicitly
include Accept. Browsers will not perform a CORS preflight for requests
containing an Accept request header.

Related: https://github.com/matrix-org/matrix-doc/pull/3225



Signed-off-by: Michael[tm] Smith <mike@w3.org>