Add tests for invalid product and quantity
Add item only if product and quantity are valid
Add test for Cart#remove
Add Cart#remove
Add another test for Cart#remove
Check Item#product_id instead of Item#product.id
Add tests for item merging
Merge items with the same product
Add Delayed Job gem
Add tests for Cart#deactivate
Add Cart#deactivate
Remove puts
Add test for queueing a delayed_job
Schedule a delayed_job after saving
Add Timecop gem
Add tests for delayed job
Reschedule delayed jobs on save