commerce_customer_field_widget_form() must set #parents before calling field_attach_form()

Commerce calls field_attach_form() on a nested structure in only two places: commerce_cart_add_to_cart_form() and commerce_customer_field_widget_form().
Now, commerce_cart_add_to_cart_form() sets #parents as expected, but commerce_customer_field_widget_form() doesn't, which means that any commerce customer profile field widget (like the addressfield one) doesn't have accurate parents set, and is unable to access data in form state.

This breaks addressfield 7.x-1.0-rc1 on the order add/edit screen.