Add HTTP::Cookie#dot_domain.
CookiestxtSaver: Prefix the domain field with a dot if the domain flag is on.

The leading dot itself should be meaningless since there is the second
field to tell if the cookie is issued for a domain and that was the
reason I once removed it, but on second thought it doesn't hurt to put
one.
CookiestxtSaver: Hide HttpOnly cookies from old browsers.

This trick is the one that has been used in Mozilla before switching
the cookie database to SQLite.