spree backend user session fallback root_path to admin_path

Author: bhtabor

lib/controllers/backend/spree/admin/user_sessions_controller.rb

@@ -34,6 +34,10 @@ def authorization_failure
     return redirect_to spree.admin_login_path
   end
 
+  def root_path
+    Spree.admin_path
+  end
+
   protected
 
   def translation_scope