Use URI.join for generate asset URL (#2741)

* Use URI.join for generate asset URL

* to String

app/helpers/routing_helper.rb

@@ -12,6 +12,6 @@ module RoutingHelper
   end
 
   def full_asset_url(source)
-    Rails.configuration.x.use_s3 ? source : File.join(root_url, ActionController::Base.helpers.asset_url(source))
+    Rails.configuration.x.use_s3 ? source : URI.join(root_url, ActionController::Base.helpers.asset_url(source)).to_s
   end
 end