BUG/MINOR: h2: forbid 'Z' as well in header field names checks

BUG/MINOR: h2: forbid 'Z' as well in header field names checks

The current tests in h2_make_htx_request(), h2_make_htx_response()
and h2_make_htx_trailers() check for an interval between 'A' and 'Z'
for letters in header field names that should be forbidden, but
mistakenly leave the 'Z' out of the forbidden range, resulting in it
being implicitly valid.

This has no real consequences but should be fixed for the sake of
protocol validity checking.

This must be backported to all relevant versions.