Support deeper levels of subdomains according to RFC 6265

The existing code for cookie validation was written 9 years ago, before the [RFC 6265](https://tools.ietf.org/html/rfc6265). 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*.