Stub has_attribute? to comply with rails 4.0.4

See https://github.com/rails/rails/commit/eaf0d50

Hey guys sorry for the lack of specs here or if this doesn't make sense (will try to look into the specs). Let me know if this is not the way we should be going.

Noticed these kinds of errors while upgrading Spree to rails 4.0.4 and thought it might be handy to have this stub here too. You can find some more context here: https://github.com/spree/spree/commit/2e8fee9aa1f0e6d0a2969db63ff12f5e487805ab#commitcomment-5693277

cheers