Use internal instance variable naming scheme for mounted URL helper proxies

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -319,7 +319,7 @@ def define_mounted_helper(name)
 
         MountedHelpers.class_eval(<<-RUBY, __FILE__, __LINE__ + 1)
           def #{name}
-            @#{name} ||= _#{name}
+            @_#{name} ||= _#{name}
           end
         RUBY
       end

actionpack/test/dispatch/prefix_generation_test.rb

@@ -57,6 +57,7 @@ def self.routes
             get "/polymorphic_path_for_engine", :to => "outside_engine_generating#polymorphic_path_for_engine"
             get "/polymorphic_with_url_for", :to => "outside_engine_generating#polymorphic_with_url_for"
             get "/conflicting_url", :to => "outside_engine_generating#conflicting"
+            get "/ivar_usage", :to => "outside_engine_generating#ivar_usage"
             root :to => "outside_engine_generating#index"
           end
 
@@ -125,6 +126,11 @@ def polymorphic_with_url_for
       def conflicting
         render :text => "application"
       end
+
+      def ivar_usage
+        @blog_engine = "Not the engine route helper"
+        render :text => blog_engine.post_path(:id => 1)
+      end
     end
 
     class EngineObject
@@ -203,6 +209,11 @@ def setup
       assert_equal "http://example.org/awesome/blog/posts/1", last_response.body
     end
 
+    test "[APP] instance variable with same name as engine" do
+      get "/ivar_usage"
+      assert_equal "/awesome/blog/posts/1", last_response.body
+    end
+
     # Inside any Object
     test "[OBJECT] proxy route should override respond_to?() as expected" do
       assert_respond_to blog_engine, :named_helper_that_should_be_invoked_only_in_respond_to_test_path