MINOR: http/hardening: implement hardening "reject-lowercase-method"

MINOR: http/hardening: implement hardening "reject-lowercase-method"

Rejects requests whose method contains a lower-case character. No
standard HTTP method uses lower-case characters and these ones are
essentially used in desynchronization attack scenarios, by trying to
exploit different case matching schemes along the chain. This is
recommended on untrusted listeners. This is implemented directly in the
h1 parser, as well as in http_method_has_forbidden_char() for h2 and
h3. H2 now uses this function instead of rolling its own loop.