- The definition of the result type of a type_date function didn't
  include INTERVAL_WEEK
- This patch adds an explicit test for INTERVAL_WEEK which results 
  in the result type for an item_date_add_intervall operation 
  being DATE rather than DATETIME when one parameter is
  INTERVAL_WEEK.


sql/item_timefunc.cc:
  Added explicit test for INTERVAL_WEEK to evaluate as MYSQL_TYPE_DATE