[BUGFIX] Customer always gets correct shipping

[rintisch]

If shipping costs differs between two countries the customer could get wrong shipping costs by choosing another country in the shipping address and afterwards checking the checkbox "shipping is same as billing".

This is fixed by using the billing country for the shipping costs when the customer chooses "shipping is same as billing".

Furthermore the checkbox is always correct checked by augmenting the condition. Before the checkbox was not check on first load but the shipping address fields were not shown.

Fixes #363