Watch currentShippingMethod instead of shipping_method

[indykoning]

CurrentShippingMethod will update on changes in checkout.shipping_methods and checkout.shipping_method
Because checkout.shipping_method was being watched while currentShippingMethod is being used in selectShippingMethod(), a race condition could occur where checkout.shipping_methods was empty while checkout.shipping_method was filled.

Resulting in currentShippingMethod.carrier_code throwing an error breaking the shipping-information API request.

By watching currentShippingMethod we are also waiting for the shipping_methods to be filled before attempting to update the shipping method