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.

actionpack/lib/action_dispatch/http/url.rb

@@ -124,11 +124,11 @@ def normalize_host(_host, options)
           return _host unless named_host?(_host)
 
           tld_length = options[:tld_length] || @@tld_length
-          subdomain  = options[:subdomain]
+          subdomain  = options.fetch :subdomain, true
           domain     = options[:domain]
 
           host = ""
-          if subdomain == true || !options.key?(:subdomain)
+          if subdomain == true
             return _host if domain.nil?
 
             host << extract_subdomain(_host, tld_length)