http2: simplify te header validation, add test

This simplifies validation of te header and adds a test case
in which array with two values is passed to param value in
isIllegalConnectionSpecificHeader

PR-URL: https://github.com/nodejs/node/pull/16246
Refs: https://github.com/nodejs/node/issues/14985


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>