This makes them more changeable and robust. Ruby will raise
NoMethodError on typos while it'll silently create a new ivar without
us noticing. Also, in my experience, a reader method gives more room to
future refactorings and eases testing because methods are easier to
stub.
The controller already does so, then, we can pass it to the service and
avoid that extra round-trip to the DB and save some memory. Spree::Order
is a rather bulky object (God object code smell perhaps) and it'll
surely make a difference.
