Send appropriate Vary header

Fixes an issue where the XHR response was sometimes
displayed when navigating back to a pushState
based history entry rather than the full page.

app/controllers/application_controller.rb

@@ -423,6 +423,7 @@ class ApplicationController < ActionController::Base
   end
 
   def map_layout
+    response.headers['Vary'] = 'X-Requested-With'
     request.xhr? ? false : 'map'
   end