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.