Fix step increment when changing shipping methods

[nielslange]

As reported in https://wordpress.org/support/topic/step-increases-by-one-for-every-shipping-method-change/, when the after the shipping method was changed, the step increment does not work correctly.

Steps to reproduce

1. Add a product to the cart and go to the cart page.
2. Verify that the quantity will be increased by 1.
3. Change the shipping method.
4. Verify that the quantity will be increased by 2.
5. Change the shipping method again.
6. Verify that the quantity will be increased by 3.

Every time the shipping method gets changed, the step quantity gets incremented.

Expected behaviour

Changing the shipping method should not affect the step quantity.

[nielslange]

Fixed with #89.