Initialize on_demand checkbox value

app/controllers/spree/admin/products_controller.rb

@@ -34,6 +34,7 @@ module Spree
           else
             # Re-fill the form with deleted params on product
             @on_hand = request.params[:product][:on_hand]
+            @on_demand = request.params[:product][:on_demand]
             render :new
           end
         end

app/views/spree/admin/products/new.html.haml

@@ -83,7 +83,7 @@
           = f.field_container :on_demand do
             = f.label :on_demand, t(".on_demand")
             %br/
-            = f.check_box :on_demand
+            = f.check_box :on_demand, value: '1', checked: @on_demand == '1'
             = f.error_message_on :on_demand
 
       .sixteen.columns.alpha