[http_parser] parseHttpDate does not apply RFC 850 two-digit year rule

[medz]

Package: http_parser

Description

parseHttpDate() supports obsolete RFC 850 HTTP-date values, but it interprets the two-digit year as 1900 + yy unconditionally.

That does not match the HTTP-date recipient rule for rfc850-date. RFC 9110 Section 5.6.7 says:

Recipients of a timestamp value in rfc850-date format, which uses a two-digit year, MUST interpret a timestamp that appears to be more than 50 years in the future as representing the most recent year in the past that had the same last two digits.

Reference: https://www.rfc-editor.org/rfc/rfc9110.html#section-5.6.7

Reproduction

With http_parser 4.1.2:

import 'package:http_parser/http_parser.dart';

void main() {
  print(parseHttpDate('Sunday, 21-Jun-26 07:28:00 GMT').toUtc());
}

Actual behavior

The value is parsed as year 1926, because the implementation currently does:

final year = 1900 + _parseInt(scanner, 2);

Current source on master:
https://github.com/dart-lang/http/blob/master/pkgs/http_parser/lib/src/http_date.dart#L71

Expected behavior

When parsed in 2026, Sunday, 21-Jun-26 07:28:00 GMT should resolve to 2026-06-21T07:28:00Z, not 1926.

More generally, rfc850-date two-digit years need to be resolved using the RFC 9110 50-year rule rather than a fixed 1900 + yy mapping.

Impact

Downstream HTTP clients that rely on parseHttpDate() for fields such as Retry-After can treat a future RFC 850 timestamp as a past timestamp and retry immediately.