Fix .subdomain regression.

actionpack/lib/action_dispatch/http/url.rb

@@ -165,7 +165,7 @@ def subdomains(tld_length = @@tld_length)
       # such as 2 to catch <tt>["www"]</tt> instead of <tt>"www.rubyonrails"</tt>
       # in "www.rubyonrails.co.uk".
       def subdomain(tld_length = @@tld_length)
-        subdomains(tld_length)
+        subdomains(tld_length).join(".")
       end
     end
   end

actionpack/test/dispatch/request_test.rb

@@ -137,30 +137,39 @@ def url_for(options = {})
   test "subdomains" do
     request = stub_request 'HTTP_HOST' => "www.rubyonrails.org"
     assert_equal %w( www ), request.subdomains
+    assert_equal "www", request.subdomain
 
     request = stub_request 'HTTP_HOST' => "www.rubyonrails.co.uk"
     assert_equal %w( www ), request.subdomains(2)
+    assert_equal "www", request.subdomain(2)
 
     request = stub_request 'HTTP_HOST' => "dev.www.rubyonrails.co.uk"
     assert_equal %w( dev www ), request.subdomains(2)
+    assert_equal "dev.www", request.subdomain(2)
 
     request = stub_request 'HTTP_HOST' => "dev.www.rubyonrails.co.uk", :tld_length => 2
     assert_equal %w( dev www ), request.subdomains
+    assert_equal "dev.www", request.subdomain
 
     request = stub_request 'HTTP_HOST' => "foobar.foobar.com"
     assert_equal %w( foobar ), request.subdomains
+    assert_equal "foobar", request.subdomain
 
     request = stub_request 'HTTP_HOST' => "192.168.1.200"
     assert_equal [], request.subdomains
+    assert_equal "", request.subdomain
 
     request = stub_request 'HTTP_HOST' => "foo.192.168.1.200"
     assert_equal [], request.subdomains
+    assert_equal "", request.subdomain
 
     request = stub_request 'HTTP_HOST' => "192.168.1.200.com"
     assert_equal %w( 192 168 1 ), request.subdomains
+    assert_equal "192.168.1", request.subdomain
 
     request = stub_request 'HTTP_HOST' => nil
     assert_equal [], request.subdomains
+    assert_equal "", request.subdomain
   end
 
   test "standard_port" do