Cannot convert the charset of a GROUP_CONCAT result:

item_sum.cc:
  "result" character set was not set into proper value.

func_gconcat.result, func_gconcat.test:
  Fixing tests accordingly.


sql/item_sum.cc:
  Bug #12829
  Cannot convert the charset of a GROUP_CONCAT result:
  "result" character set was not set into proper value.
mysql-test/t/func_gconcat.test:
  Bug #12829
mysql-test/r/func_gconcat.result:
  Bug #12829