Fix for asset loading issue:

When assets have been copied with rake admin:copy_assets, then rails_admin should NOT insert
another ActionDispatch::Static middleware, because it eclipses Rails's native middleware and assets
are loaded from rails_admin's public folder instead of the application's public folder.
Fixed by checking if any of the rails_admin subdirectories exist in any of the static asset classes
(images, javascripts, stylesheets).
Also, if the middleware is inserted, it should be done AFTER, not IN FRONT of Rails's own static asset
middleware, to permit precedence to the application's public folder over the gem's.

lib/rails_admin/engine.rb

@@ -4,8 +4,12 @@
 module RailsAdmin
   class Engine < Rails::Engine
     initializer "static assets" do |app|
-      if app.config.serve_static_assets
-        app.middleware.insert 0, ::ActionDispatch::Static, "#{root}/public"
+
+      # If assets were copied for app's public/ folder, e.g. via rake admin:copy_assets, then don't insert the static
+      # asset server, as it iterferes with Rails's own asset server.
+      if app.config.serve_static_assets &&
+         !(%w(stylesheets images javascripts).any?{|cat| File.exists?(File.join(Rails.root,'public',cat,'rails_admin'))})
+        app.middleware.insert_after ::ActionDispatch::Static, ::ActionDispatch::Static, "#{root}/public"
       end
     end