Rails 4 tweaks for Spree API component

Core build still looks all green on Rails 4 rc2. API component is down to 1 failing spec. Frontend should be all green too in rc2 but I haven't run it yet. We've got some more backend specs failing but I believe that's related to some spec environment tweak only. Users should be able to use backend just fine in general.

I'm excited with the possibility that we may have Spree just ready for Rails 4 as soon as it's released :)