remove warnings about @variable_for_layout

Signed-off-by: José Valim <jose.valim@gmail.com>

actionpack/test/controller/render_test.rb

@@ -25,6 +25,8 @@ def new
 class TestController < ActionController::Base
   protect_from_forgery
 
+  before_filter :set_variable_for_layout
+
   class LabellingFormBuilder < ActionView::Helpers::FormBuilder
   end
 
@@ -364,17 +366,14 @@ def render_action_hello_world_as_symbol
   end
 
   def layout_test_with_different_layout
-    @variable_for_layout = nil
     render :action => "hello_world", :layout => "standard"
   end
 
   def layout_test_with_different_layout_and_string_action
-    @variable_for_layout = nil
     render "hello_world", :layout => "standard"
   end
 
   def layout_test_with_different_layout_and_symbol_action
-    @variable_for_layout = nil
     render :hello_world, :layout => "standard"
   end
 
@@ -383,7 +382,6 @@ def rendering_without_layout
   end
 
   def layout_overriding_layout
-    @variable_for_layout = nil
     render :action => "hello_world", :layout => "standard"
   end
 
@@ -666,8 +664,11 @@ def render_with_filters
 
   private
 
+    def set_variable_for_layout
+      @variable_for_layout = nil
+    end
+
     def determine_layout
-      @variable_for_layout ||= nil
       case action_name
         when "hello_world", "layout_test", "rendering_without_layout",
              "rendering_nothing_on_layout", "render_text_hello_world",