fixing https://github.com/collectiveidea/acts_as_audited/issues/#issue/35
fixing https://github.com/collectiveidea/acts_as_audited/issues/#issue/35

accessible_attributes never returns nil.

xuwupeng2000/audited,azanar/audited,cfiorini/audited,Datacom/audited,ledermann/audited,ontrack/audited,happio/audited,collectiveidea/audited,GhostGroup/audited,klatunetworks/audited,ktheory/acts_as_audited,airbnb/audited,instacart/audited,lrgalego/audited,ConsultingMD/audited,reenhanced/audited,ajoulie/audited,ZHANGJIE982/audited,ishail/audited,simplepractice/audited