Give access to default value checkout field and change woocommerce_form_field()

Woocommerce seems, all calculation shipping methods based state, country and postal code. Here in Indonesia and most ASEAN country using city as their based calculation.

And we are don't know other country how they calculation shipping. So, we must allow some custom default value for checkout field. Here, I change WC_Checout::get_value() and woocommerce_form_field.

May be, the code I changed still need improving