Only cache the view_context_class in one place

This patch removes the instance writer of view_context_class.
Subclasses may override it, but it doesn't need to be written.  This
also eliminates the need to cache the return value of the class level
`view_context_class` method.

actionpack/lib/action_dispatch/testing/assertions/routing.rb

@@ -160,9 +160,16 @@ module ActionDispatch
           @controller.singleton_class.include(_routes.url_helpers)
 
           if @controller.respond_to? :view_context_class
-            @controller.view_context_class = Class.new(@controller.view_context_class) do
+            view_context_class = Class.new(@controller.view_context_class) do
               include _routes.url_helpers
             end
+
+            custom_view_context = Module.new {
+              define_method(:view_context_class) do
+                view_context_class
+              end
+            }
+            @controller.extend(custom_view_context)
           end
         end
         yield @routes

actionview/lib/action_view/rendering.rb

@@ -55,10 +55,8 @@ module ActionView
       end
     end
 
-    attr_internal_writer :view_context_class
-
     def view_context_class
-      @_view_context_class ||= self.class.view_context_class
+      self.class.view_context_class
     end
 
     # An instance of a view class. The default view class is ActionView::Base.