Hstore columns should always be saved, if changed or not

According to issue: https://github.com/rails/rails/issues/6127 changes to Hstore columns were not saved unless marked with __*_will_change!__. This Pull Request is going to fix that by introducing a list of columns that should always be saved if their were changed or not. 

__dirty_attribute_names__ is a class variable added to __dirty.rb__.
If a column name will be added to that list all then AR will always update that attribute. 

Potentially the __serialized_attributes__ may be managed with __dirty_attribute_names__ so we can have only one way to deal with those things. 

Tested over ruby-2.0.0-p0 and postgresql 9.2.3.