Also, Item_sum_hybrid->charset was removed as redundant,
and switched to use collation.collation instead.


mysql-test/r/func_group.result:
  Bug #6658 MAX(column) returns incorrect coercibility
mysql-test/r/func_str.result:
  Bug #6658 MAX(column) returns incorrect coercibility
mysql-test/t/func_group.test:
  Bug #6658 MAX(column) returns incorrect coercibility
sql/item_func.cc:
  Bug #6658 MAX(column) returns incorrect coercibility
sql/item_sum.cc:
  Bug #6658 MAX(column) returns incorrect coercibility
sql/item_sum.h:
  Bug #6658 MAX(column) returns incorrect coercibility