Bug#21811 Odd casting with date + INTERVAL arithmetic

- 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

@@ -1876,7 +1876,7 @@ void Item_date_add_interval::fix_length_and_dec()
     cached_field_type= MYSQL_TYPE_DATETIME;
   else if (arg0_field_type == MYSQL_TYPE_DATE)
   {
-    if (int_type <= INTERVAL_DAY || int_type == INTERVAL_YEAR_MONTH)
+    if (int_type <= INTERVAL_DAY || int_type == INTERVAL_YEAR_MONTH || int_type == INTERVAL_WEEK )
       cached_field_type= arg0_field_type;
     else
       cached_field_type= MYSQL_TYPE_DATETIME;