fixed bug in Item_sum
fixed bug in dependence remover
after merge fix


mysql-test/r/subselect.result:
  after merge fix
mysql-test/r/union.result:
  new test
mysql-test/t/subselect.test:
  after merge fix
mysql-test/t/union.test:
  new test
sql/item.cc:
  fixed returned value
sql/item.h:
  fixed flag dropper
  (I was not able to find Item whicj need more to be fix_fielded twice)
sql/item_subselect.h:
  fixed initialisation
sql/item_sum.cc:
  fixed absence of walk method of Item_sum
sql/item_sum.h:
  fixed absence of walk method of Item_sum
sql/mysql_priv.h:
  setup_fields reverter
sql/sql_base.cc:
  setup_fields reverter
sql/sql_derived.cc:
  fixed bug of lack of fix_fields call
sql/sql_union.cc:
  fixed bug of lack of fix_fields call