- 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.