I was digging into Spree Promotions to for a clien to turn promotions
on/off for specific products.
While doing so, I found that the PercentPerItem maps over an array of
Spree::Product Objects in order to make the Promotional Adjustment rather
than only compairing the thhe Spree::Product#id for a match.
See this,
https://github.com/spree/spree/blob/master/core/app/models/spree/calculator/percent_per_item.rb#L40,
for more code.
Is there a reason for this? If not, I'll put a pull request to compare the
Object ids rather than the whole ruby Object.
