fix for cast( AS DATETIME) + 0 operation.
  I just implemented Item_datetime_typecast::val() method
  as it is usually done in other classes.
  Should be fixed more radically in 5.0


mysql-test/r/type_datetime.result:
  result added
mysql-test/t/type_datetime.test:
  testcase
sql/item_timefunc.h:
  added double conversion to Item_datetime_typecast