aggregated in outer context returned wrong results.
This happened only if the subquery did not contain any references
to outer fields.
As there were no references to outer fields the subquery erroneously
was taken for non-correlated one.
Now any set function aggregated in outer context makes the subquery
correlated.


mysql-test/r/subselect.result:
  Added a test case for bug #27257.
mysql-test/t/subselect.test:
  Added a test case for bug #27257.