http2: correct behaviour for enablePush unpack

The only valid values for enablePush are 0 and 1. If validation
is requested, we should verify that it wasn't set to another
value rather than casting to Boolean regardless of value.

PR-URL: https://github.com/nodejs/node/pull/15167


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>