fix: preserve original case for DSN query parameter values

[tank-500m]

Summary

Fix #1800

strings.ToLower was applied to query parameter values for true/false boolean comparison, but the lowered value leaked into the default branch, causing settings like log_comment to be unintentionally lowercased.

Checklist

Delete items not relevant to your PR:

• Unit and integration tests covering the common scenarios were added
• A human-readable description of the changes was provided to include in CHANGELOG
• For significant changes, documentation in https://github.com/ClickHouse/clickhouse-docs was updated with further explanations or tutorials