SCRUM
BUG
correct wrong code in group_concat


mysql-test/r/func_gconcat.result:
  add new tests
mysql-test/t/func_gconcat.test:
  add new tests
sql/field.cc:
  add new variable for group_concat
sql/field.h:
  add new variable for group_concat
sql/item_sum.cc:
  correct wrong code
sql/item_sum.h:
  correct wrong code