The end_update() function uses the Item::save_org_in_field() function to
save original values of items into the group buffer. But for the 
Item_func_set_user_var this method was mapped to the save_in_field method.
The latter function wrongly decides to use the result_field. This leads to
saving incorrect value in the grouping buffer and wrong result of the whole
query.

The can_use_result_field argument of the bool type is added to the
Item_func_set_user_var::save_in_field() function. If it is set to FALSE
then the item's result field won't be used. Otherwise it will be detected
whether the result field will be used (old behaviour).
Two wrapping functions for the function above are added to the 
Item_func_set_user_var class:
the save_in_field(Field *field, bool no_conversions) - it calls the above
function with the can_use_result_field set to TRUE.
the save_org_in_field(Field *field) - same, but the can_use_result_field
is set to FALSE.


mysql-test/t/user_var.test:
  Added a test case for the bug#28494: Grouping by Item_func_set_user_var
  produces incorrect result.
mysql-test/r/user_var.result:
  Added a test case for the bug#28494: Grouping by Item_func_set_user_var
  produces incorrect result.
sql/item_func.cc:
  Bug#28494: Grouping by Item_func_set_user_var produces incorrect result.
  The can_use_result_field argument of the bool type is added to the
  Item_func_set_user_var::save_in_field() function. If it is set to FALSE
  then the item's result field won't be used. Otherwise it will be detected
  whether the result field will be used (old behaviour).
sql/item_func.h:
  Bug#28494: Grouping by Item_func_set_user_var produces incorrect result.
  The can_use_result_field argument of the bool type is added to the
  Item_func_set_user_var::save_in_field() function.
  Two wrapping functions for the function above are added to the 
  Item_func_set_user_var class:
  the save_in_field(Field *field, bool no_conversions) - it calls the above
  function with the can_use_result_field set to TRUE.
  the save_org_in_field(Field *field) - same, but the can_use_result_field
  is set to FALSE.