Add frontend/backend/api decorators to autoload path

This is required in order to have the decorators correctly
unloaded when we reload the application (calling reload! in
console or on each request in development).

lib/solidus_support/engine_extensions.rb

@@ -59,10 +59,13 @@ def enable_solidus_engine_support(engine)
         paths['app/controllers'] << "lib/controllers/#{engine}"
         paths['app/views'] << "lib/views/#{engine}"
 
+        path = root.join("lib/decorators/#{engine}")
+
+        config.autoload_paths += path.glob('*')
+
         engine_context = self
         config.to_prepare do
           engine_context.instance_eval do
-            path = root.join("lib/decorators/#{engine}")
             load_solidus_decorators_from(path)
           end
         end