fix default timezone

[jlahtinen]

No description provided.

[yahonda]

Thank you for the pull request.

This change would replace the ActiveRecord default_timezone lookup with a check on the connection's session time zone. There are a couple of concerns:

• The current code intentionally reads ActiveRecord.default_timezone (:utc or :local) so that ruby-plsql stays consistent with how ActiveRecord interprets database values. This PR would break that integration.
• The session time zone (connection.time_zone) and default_timezone are different concepts. A session could be set to "Asia/Tokyo" while the application still wants default_timezone: :utc.
• The existing test "should use ActiveRecord default_timezone as default" in schema_spec.rb would fail with this change.

Could you describe the problem you were trying to solve? If there is a specific scenario where the timezone behavior is incorrect, we would be happy to look into an alternative fix.