- Type casting was not consequent, thus when adding a DATE type with
  a WEEK interval the result tpe was DATETIME and not DATE as is the
  norm.
- By changing the order of the date type enumerations the type casting
  bug is resolved. To comply with the new order the array 
  interval_type_to_name needed to change accordingly.


include/my_time.h:
  Changed enumeration order to make week interval appear in a more
  logical order. This affects type casting when a date is added to an
  interval.
sql/time.cc:
  Moved string WEEK to match enumeration in interval_type