A change was made recently to spree for multi-currency.

The copy_price method used to return the price:
https://github.com/spree/spree/commit/58ec5cd5124e66c60811a075b290248962159f9e#L12L27

but it no longer does, so we must not rely on its return value in our 'overridden' method.

fixes logic for changing price after quantity changes

Related to #40
fixes logic for changing price after quantity changes

Fixes #40