Fix wrong HTTP_HOST in rack session

Webrat relies on www.example.com while Rack::Test expects example.org

lib/webrat/rack.rb

@@ -1,5 +1,7 @@
 require "rack/test"
 
+Rack::Test::DEFAULT_HOST.replace("www.example.com")
+
 module Webrat
   class RackSession < Session
     def initialize(rack_test_session) #:nodoc:

spec/integration/rack/test/webrat_rack_test.rb

@@ -17,10 +17,10 @@ class WebratRackTest < Test::Unit::TestCase
     assert_equal "/", last_request.env["PATH_INFO"]
   end
 
-  # def test_redirects
-  #   visit "/redirect_absolute_url"
-  #   assert_equal "spam", response_body
-  # end
+  def test_redirects
+    visit "/redirect_absolute_url"
+    assert_equal "spam", response_body
+  end
 
   def test_assertions_after_visit
     visit "/"