correct bug 1085 (a problem with min/max functions)
add tests of bugs 833,836,1085


mysql-test/r/func_gconcat.result:
  add tests of bugs 833, 836, 1085
mysql-test/t/func_gconcat.test:
  add tests of bugs #833, 836, 1085
sql/item_sum.cc:
  merge
sql/item_sum.h:
  correct bug 1085