Fix max-stale in default refresh_pattern (#1664)

Fix max-stale in default refresh_pattern (#1664)

RefreshPattern constructor must set data fields to honor refresh_pattern
defaults promised in squid.conf.documented. For max-stale, that implies
making max_stale negative. A negative value allows refreshCheck() to use
a max_stale directive value (i.e. Config.maxStale that defaults to 1
week). The buggy constructor set max_stale to 0 instead and, hence,
refreshCheck() ignored max_stale directive when no refresh_pattern rules
were configured.

The fixed bug did not affect Squids configured using explicit
refresh_pattern rules because those rules are handled by
parse_refreshpattern() which sets max_stale to -1 by default. Our
squid.conf.default does have explicit refresh_pattern rules.