Shifted SaleLine cart data into `serialize()` method. #6409

1. A method call now returns SaleLine data for each line in cart.
2. Added a `serialize()` method to `product.product` which returns
GA-specific product data if purpose is set to 'ga'.
2. Added test.
Merge pull request #68 from PritishC/feature/task-6409

Shifted SaleLine cart data into `serialize()` method. #6409