Ar preserve hashes

reopen of https://github.com/attr-encrypted/attr_encrypted/pull/117
@tamird,

It looks good but I am unsure why you moved this method definition out of the if/else block. Your new tests still pass even if this change isn't included. I tested with activerecord 3.1 and 3.2 so I should have hit both conditions.

If it isn't def necessary then I think we should remove it.