use #send() to not raise error #70

In Rails 4.0.x the AM::Dirty methods are private, thus the error

> private method `attribute_was' called

is raised when using #was or #changed? on the accessor. Fixed this
by making use uf #send to call the private methods. Starting with
4.1 these methods are public.
Merge pull request #71 from lwe/lwe-70-dirty

use #send() to not raise error #70