With ALLOWED_ORIGINS set, every request carrying an Origin that was not on
the list was failed with an unhandled error, which the exception filter
answered with a 500. A browser sends Origin on every POST and WebSocket
upgrade, including same-origin ones, so opening TREK under any address the
list did not name (the LAN IP, a second hostname, http instead of https)
broke the login with "Internal Server Error" while another browser that
used the listed address worked.
A request whose Origin is the host it was sent to is now treated as
same-origin and always passes, for REST and the /ws upgrade alike. A
foreign origin is still refused, now with a 403 "Not allowed by CORS" and a
warning in the log naming the origin to add.
Closes#2543
Maurice