These changes add consistent behavior when converting expired cookies back and forth from string representation into Symfony\Component\HttpFoundation\Cookie instances in Cookie::fromString:

• When Max-Age is zero and expires is in the past, the expires date is kept as is (previous behavior: expires is overwritten with current timestamp because it is reset to current timestamp + Max-Age)
• When Max-Age is zero and expires is in the future, expires is reset to current timestamp, as Max-Age is the preferred "source of truth" (same as previous behavior)
• Add tests for how the Cookie class handles Max-Age in a cookie string and how expires and Max-Age interact
• Extract helper function expiresTimestamp so converting to a unix timestamp can be reused in Cookie::fromString

This is more a new feature than a bug fix in my mind, therefore I would include it in 5.1+.