http2: refactor counter in `mapToHeaders` inner loop

This cosmetic 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. Got rid of old-style var declaration. It does make sense to sometimes use var in for loops for optimization, but not in this context. Also changed post-increment to pre-increment in iteration steps. It is probably done by the optimizer anyway, otherwise it will save an opcode each iteration. And it is a good practice.

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines