reduce calls to `named_host?`

`normalize_host` already calls `named_host?`, so there is no reason to
test `named_host?` again in the `extract_domain` method.
only extract domain from the options hash once
lookup subdomain from the options hash once, defaulting to true

if the subdomain wasn't specified, it's the same as if specifying
:subdomain as `true`, so we can default the value to `true` safely.