introduced during implementation of TIMESTAMP columns, which are able 
to store NULLs (Unfortunately it is impossible to write test case for
this. Kudos to Holyfoot for noticing it!)


sql/field.cc:
  Field_timestamp::get_auto_set_type(): To preserve compatibility 
  with pre-4.1 we have to handle TIMESTAMP_OLD_FIELD columns properly. 
  (It is ok to handle them as TIMESTAMP_DNUN_FIELD since value of first
   such column should be set to current timestamp on both INSERT and UPDATE
   operations, and we don't call this method for rest of them).