Bug #21531: EXPORT_SET() doesn't accept args with coercible character sets

- Fix typo in Item_func_export_set::fix_length_and_dec() which caused character set aggregation to fail
- Remove default argument from last arg of agg_arg_charsets() function, to reduce potential errors

mysql-test/r/func_misc.result

@@ -93,3 +93,7 @@ SELECT IS_USED_LOCK('bug16501');
 IS_USED_LOCK('bug16501')
 NULL
 DROP TABLE t1;
+select export_set(3, _latin1'foo', _utf8'bar', ',', 4);
+export_set(3, _latin1'foo', _utf8'bar', ',', 4)
+foo,foo,bar,bar
+End of 4.1 tests

mysql-test/t/func_misc.test

@@ -83,4 +83,9 @@ connection default;
 
 DROP TABLE t1;
 
-# End of 4.1 tests
+#
+# Bug #21531: EXPORT_SET() doesn't accept args with coercible character sets
+#
+select export_set(3, _latin1'foo', _utf8'bar', ',', 4);
+
+--echo End of 4.1 tests

sql/item_func.h

@@ -156,8 +156,7 @@ class Item_func :public Item_result_field
     return agg_item_collations_for_comparison(c, func_name(),
                                               items, nitems, flags);
   }
-  bool agg_arg_charsets(DTCollation &c, Item **items, uint nitems,
-                        uint flags= 0)
+  bool agg_arg_charsets(DTCollation &c, Item **items, uint nitems, uint flags)
   {
     return agg_item_charsets(c, func_name(), items, nitems, flags);
   }

sql/item_strfunc.cc

@@ -2624,8 +2624,8 @@ void Item_func_export_set::fix_length_and_dec()
   uint sep_length=(arg_count > 3 ? args[3]->max_length : 1);
   max_length=length*64+sep_length*63;
 
-  if (agg_arg_charsets(collation, args+1, min(4,arg_count)-1),
-                       MY_COLL_ALLOW_CONV)
+  if (agg_arg_charsets(collation, args+1, min(4,arg_count)-1,
+                       MY_COLL_ALLOW_CONV))
     return;
 }