incorrect TLD

```
host = DomainName("example.co.uk-")
host.tld
=> 'uk-'  # not a valid TLD

host = DomainName("example.co.uk ")
host.tld
=> 'uk '  # not a valid TLD (space at the end)

host = DomainName("a.a-a")
host.tld
=> 'a-a '  # not a valid TLD
```