modified fix #1872 according to jose valim´s suggestions

actionpack/lib/action_controller/metal/testing.rb

@@ -4,6 +4,11 @@ module Testing
 
     include RackDelegation
 
+    def recycle!
+      @_url_options = nil
+    end
+
+
     # TODO: Clean this up
     def process_with_new_base_test(request, response)
       @_request = request

actionpack/lib/action_controller/test_case.rb

@@ -447,6 +447,7 @@ def process(action, parameters = nil, session = nil, flash = nil, http_method =
         @controller.params.merge!(parameters)
         build_request_uri(action, parameters)
         @controller.class.class_eval { include Testing }
+        @controller.recycle! 
         @controller.process_with_new_base_test(@request, @response)
         @assigns = @controller.respond_to?(:view_assigns) ? @controller.view_assigns : {}
         @request.session.delete('flash') if @request.session['flash'].blank?

actionpack/test/controller/default_url_options_with_filter_test.rb

@@ -0,0 +1,29 @@
+require 'abstract_unit'
+
+
+class ControllerWithBeforeFilterAndDefaultUrlOptions < ActionController::Base
+
+  before_filter { I18n.locale = params[:locale] }
+  after_filter { I18n.locale = "en" }
+
+  def target
+    render :text => "final response"
+  end
+
+  def redirect
+    redirect_to :action => "target"
+  end
+
+  def default_url_options
+    {:locale => "de"}
+  end 
+end
+
+class ControllerWithBeforeFilterAndDefaultUrlOptionsTest < ActionController::TestCase
+
+  # This test has it´s roots in issue #1872
+  test "should redirect with correct locale :de" do
+    get :redirect, :locale => "de"
+    assert_redirected_to "/controller_with_before_filter_and_default_url_options/target?locale=de"
+  end
+end