http2: rename counter in `mapToHeaders` inner loop

This change is to prevent potential bugs - e.g., someone might
automatically use the variable `k` instead of `key`, that is used in
vicinity of this loop.
Also changed postincrement to preincrement in iteration steps. It is
probably done by the optimizer anyway, but otherwise it will save an
opcode each iteration. And it is a good practice.

PR-URL: https://github.com/nodejs/node/pull/32012


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>