http_parser: Follow RFC-7230 Sec 3.2.4

RFC-7230 Sec 3.2.4 expressly forbids line-folding in header
field-names.

This change no longer allows obsolete line-folding between the
header field-name and the colon. If HTTP_PARSER_STRICT is unset,
the parser still allows space characters.

Reviewed-By: Fedor Indutny <fedor@indutny.com>