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Checks the origin request header against the list of allowed origins.

Should be called by transports before connecting when appropriate.
If the origin header matches the allowed origins, no origin header was
sent or no origin was configured, it will return the given connection.

Otherwise a 403 Forbidden response will be sent and the connection halted.
It is a noop if the connection has been halted.
Please help! Open an issue on GitHub if this assessment is incorrect.