In the string context the MIN() and MAX() functions don't take
into account the unsignedness of the UNSIGNED BIGINT argument
column.

I.e.:
        CREATE TABLE t1 (a BIGINT UNSIGNED);
        INSERT INTO t1 VALUES (18446668621106209655);
        SELECT CONCAT(MAX(a)) FROM t1;

returns -75452603341961.


mysql-test/r/func_group.result:
  Test case for bug #11766094.
mysql-test/t/func_group.test:
  Test case for bug #11766094.
sql/item.cc:
  Bug #11766094 - 59132: MIN() AND MAX() REMOVE UNSIGNEDNESS
  
  The Item_cache_int::val_str() method has been modified to
  take into account the unsigned_flag value when converting
  data to string.