Fixed bug #9939: a wrong conversion of arguments
  for functions COALESCE and IFNULL.
  The str_op virtual method was added into Item_func_numhybrid.
item_func.cc:
  Fixed bug #9939: a wrong conversion of arguments
  for functions COALESCE and IFNULL.
  The str_op virtual method was added into Item_func_numhybrid.
item_cmpfunc.h, item_cmpfunc.cc:
  Fixed bug #9939: a wrong conversion of arguments
  for functions COALESCE and IFNULL.
  Item_func_coalesce and Item_func_ifnull now
  inherit from a modified Item_func_numhybrid.
case.test, case.result:
  Added test cases for bug #9939.