- Type casting was not consequent, thus when adding a DATE type with
  a WEEK interval the result type was DATETIME and not DATE as is the
  norm.
- By changing the order of the date internal enumerations the deviant
  type casting is resolved (Item_date_add_interval::fix_length_and_dec()
  which determines result type for this operation assumes that addition
  of any interval with value <= INTERVAL_DAY to date value will result
  in date). There are two independant places to change:
  interval_names[] and interval_type.


mysql-test/r/func_date_add.result:
  Updated result file for type casting test
mysql-test/r/func_time.result:
  Updated result file for type casting test
mysql-test/t/func_date_add.test:
  Added test for type casting when adding intervals to date.
sql/item_timefunc.cc:
  Changed order of "week" key word to match the date interval enumeration.
sql/item_timefunc.h:
  Changed the order of the enumeration to better follow interval sizes.