This is an additional fix.
Item::val_xxx methods are supposed to use original data source and
Item::val_xxx_result methods to use the item's result field. But for the
Item_func_set_user_var class val_xxx_result methods were mapped to val_xxx
methods. This leads, in particular, to producing bad sort keys and thus
wrong order of the result set of queries with group by/order by clauses.

The set of val_xxx_result methods is added to the Item_func_set_user_var
class. It's the same as the val_xxx set of method but uses the result_field
to return a value.


mysql-test/t/user_var.test:
  Corrected test case for hte bug#28494.
mysql-test/r/user_var.result:
  Corrected test case for hte bug#28494.
sql/item_func.cc:
  Bug#28494: Grouping by Item_func_set_user_var produces incorrect result.
  The set of val_xxx_result methods is added to the Item_func_set_user_var
  class. It's the same as the val_xxx set of method but uses the result_field
  to return a value.
sql/item_func.h:
  Bug#28494: Grouping by Item_func_set_user_var produces incorrect result.
  The set of val_xxx_result methods is added to the Item_func_set_user_var
  class.