Dump UUID default functions to schema.rb [2nd version]. Fixes #10751.

This PR is second version for fixing #10751. First version is #10811.

Please see #10751 about original problem.

In master, UUID default functions are not dumped to db/schema.rb, because they are not extracted to `PostgreSQLColum`.

In first version, I modified `PostgreSQLColum.extract_value_from_default` method to return a function name string when `default` argument is uuid generator. But I've understood this was wrong approach, because `extract_value_from_default` method must extract value.