fix rails4 enums attribute issue

With a Rails 4 enum field the changed attributes hash contains the
mapped value and when prev[attr] assigns the value it will be lost.
close #384; Fix originator method so it doesn't return inappropriate cached values or inaccurate values for reified model isntances
Only run tests for ActiveRecord::Enum when it is defined
Merge branch 'enums'

Conflicts:
	CHANGELOG.md
Use hash rocket hash construction syntax in PostWithStatus model to avoid failures testing against Ruby18