Fix header parsing to compare HTTP header field names case
insensitively. This fixes the problem where responses get incorrectly
marked as errors if the response header field name case doesn't happen
to match to what is in the code.

Also compare connection and transfer-encoding tokens case insensitively.

(Reference: RFC 2616: header field names are not case sensitive.)

Co-authored-by: Stefan Bühler <stbuehler@web.de>