treated as DEFAULT NULL columns (independently from their position in
table).

(still to be discussed with Monty, Brian, Trudy et al. before push)


mysql-test/r/type_timestamp.result:
  Update test after making TIMESTAMP NULL columns without explicit 
  DEFAULT value to be always treated as DEFAULT NULL columns 
  (independently from their position in table).
mysql-test/t/type_timestamp.test:
  Update test after making TIMESTAMP NULL columns without explicit 
  DEFAULT value to be always treated as DEFAULT NULL columns 
  (independently from their position in table).
sql/sql_parse.cc:
  add_field_to_list(): made TIMESTAMP NULL columns without explicit 
  DEFAULT value to be always treated as DEFAULT NULL columns (independently
  from their position in table). This also simplifies code a bit.