Brought the domain method in AD http url inline with subdomain where...

Brought the domain method in AD http url inline with subdomain where @@tld_length is used by default. Also set the default value of @@tld_length to 1.

actionpack/lib/action_dispatch/http/url.rb

@@ -2,6 +2,7 @@ module ActionDispatch
   module Http
     module URL
       mattr_accessor :tld_length
+      self.tld_length = 1
 
       # Returns the complete URL used for this request.
       def url
@@ -67,7 +68,7 @@ def server_port
 
       # Returns the \domain part of a \host, such as "rubyonrails.org" in "www.rubyonrails.org". You can specify
       # a different <tt>tld_length</tt>, such as 2 to catch rubyonrails.co.uk in "www.rubyonrails.co.uk".
-      def domain(tld_length = 1)
+      def domain(tld_length = @@tld_length)
         return nil unless named_host?(host)
 
         host.split('.').last(1 + tld_length).join('.')

actionpack/test/dispatch/request_test.rb

@@ -96,6 +96,9 @@ class RequestTest < ActiveSupport::TestCase
     request = stub_request 'HTTP_HOST' => "www.rubyonrails.co.uk"
     assert_equal "rubyonrails.co.uk", request.domain(2)
 
+    request = stub_request 'HTTP_HOST' => "www.rubyonrails.co.uk", :tld_length => 2
+    assert_equal "rubyonrails.co.uk", request.domain
+
     request = stub_request 'HTTP_HOST' => "192.168.1.200"
     assert_nil request.domain