- 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:
  Test EXPORT_SET() with charset coersion (bug #21531)
mysql-test/t/func_misc.test:
  Test EXPORT_SET() with charset coersion (bug #21531)
sql/item_func.h:
  Remove default argument from last arg of agg_arg_charsets() function, to reduce potential errors.
sql/item_strfunc.cc:
  Fix typo in Item_func_export_set::fix_length_and_dec() which caused character set aggregation to fail.