Make DuckParser preserve identifier case

[ViggoC]

For historical reason that the old version DuckDB convert unquoted identifier to lower case, many test cases are wrote in case-insensitive way. And we have to preserve_identifier_case=false while upgrading duckdb. Since Velox itself is case-sensitive, should we set preserve_identifier_case=true, and and resolve the historical problem of case-insensitive test cases?