Prevent time field converting zones in changes 'was'

When changing "Time" attributes the old '_was' value (e.g. bonus_time_was) will be converted based on zone.  However, in all other cases "Time" attributes do not convert zones.

The expected behaviour is to NOT convert zones due to variations across dates, even within a single zone -- see more discussion here: https://github.com/rails/rails/issues/3145#issuecomment-5400841 

Original commit added here: https://github.com/rails/rails/commit/36129f21b86db4bd69e932e586129e246c2a5ca8

    Dirty datetime attributes should be aware of time zone info

Then later removed in commit slated for rails 4.(?) https://github.com/rails/rails/commit/cb400b0c654e62c99dca1faf052c9389c58d692c 

    remove unnecessary code
    it was added in 36129f2
    but isn't useful anymore as corresponding tests pass without it


I've made the PR to rails 3-2-stable since rails 4+ seems to have rearranged the dirty.rb file considerably.