Support deeper levels of subdomains according to RFC 6265

[RKushnir]

The existing code for cookie validation was written 9 years ago, before the RFC 6265. So it only supports one level of subdomains between the cookie domain-value and the requested host name:

Set-Cookie: "Hello=w0r1d;Path=/;Domain=.grandma.com"

matches eat.grandma.com, but does not match lets.eat.grandma.com.