mysql-test/r/func_group.result:
  Added some tests and fix for Bug#5615.
mysql-test/r/show_check.result:
  Fix for Bug#5615.
mysql-test/t/func_group.test:
  Added some tests.
sql/item_sum.cc:
  Fix for Bug#5615.
sql/item_sum.h:
  Fix for Bug#5615.
sql/sql_select.cc:
  Fix for Bug#5615.
sql/sql_select.h:
  Fix for Bug#5615.