Updated DirtyModel's @changed_attributes hash to be symbol/string agnostic 

DirtyModel uses a hash to keep track of any changes made to attributes
of an instance. When using the attribute_will_change! method, you must
supply is a string and not a symbol or the *_changed? method will break
(because it is looking for the attribute name as a string in the keys
of the underlying hash). To remedy this, I simply made the underlying
hash a HashWithIndifferentAccess so it won't matter if you supply
the attribute name as a symbol or string to attribute_will_change!.