prevent nil exception if the params.Shipping is nil
prevent nil exception if the params.Shipping is nil

dominicphillips/stripe-go,dominicphillips/stripe-go,stripe/stripe-go,mondough/stripe-go,mondough/stripe-go,froatsnook/stripe-go,froatsnook/stripe-go,ChannelMeter/stripe-go,stripe/stripe-go,ChannelMeter/stripe-go