Additional fix for the bug#27759: Wrong DATE/DATETIME comparison in LEAST()/GREATEST() functions.

mysql-test/t/type_datetime.test:
  Additional test case for the bug#27759.
sql/item_func.h:
  Bug#27759: Wrong DATE/DATETIME comparison in LEAST()/GREATEST() functions.
  Now the Item_func_min_max::result_as_longlong() returns TRUE when LEAST()
  compares DATE/DATETIME values. This allows caller to obtain correct integer
  values through the val_int() function.

mysql-test/t/type_datetime.test

@@ -206,6 +206,8 @@ select greatest(cast('01-01-01' as date), '01-01-02');
 select least(cast('01-01-01' as date), '01-01-02') + 0;
 select greatest(cast('01-01-01' as date), '01-01-02') + 0;
 select least(cast('01-01-01' as datetime), '01-01-02') + 0;
+select cast(least(cast('01-01-01' as datetime), '01-01-02') as signed);
+select cast(least(cast('01-01-01' as datetime), '01-01-02') as decimal);
 --disable_warnings
 DROP PROCEDURE IF EXISTS test27759 ;
 --enable_warnings

sql/item_func.h

@@ -703,6 +703,7 @@ class Item_func_min_max :public Item_func
   my_decimal *val_decimal(my_decimal *);
   void fix_length_and_dec();
   enum Item_result result_type () const { return cmp_type; }
+  bool result_as_longlong() { return compare_as_dates; };
   uint cmp_datetimes(ulonglong *value);
 };