net/http: allow zero-valued Expires in Cookie.Valid

net/http: allow zero-valued Expires in Cookie.Valid

The value of Expires is only checked when a value is set.

This fixes the bug that a cookie with a zero-valued Expire
was considered invalid, even though Expires is an optional
field.

Fixes #52989

Change-Id: I206c50e9b6ea2744a92c74673d589ce2aaa62670
Reviewed-on: https://go-review.googlesource.com/c/go/+/407654
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>