Updated test for uppercased values

added literals off and on to the testcase

added postgresql platform conversion test

Prevent Connection from maintaining a second reference to an injected PDO object. Previously, if a developer explicitly closed the Connection, only the _conn reference was destroyed, but the _params['pdo'] reference remained and kept the PDO connection alive. By unsetting the _params reference, we maintain only the _conn reference, exactly as if the PDO connection is generated internally.