added test for render context promotion and demotion.
added test for render context promotion and demotion.

sunng87/handlebars-rust,sunng87/handlebars-rust,tailhook/handlebars-rust,tailhook/handlebars-rust,ihrwein/handlebars-rust,tempbottle/handlebars-rust,tempbottle/handlebars-rust,wang19742008/handlebars-rust