Unit tests currently only cover SQLite, MySQL and PostgreSQL.

Tests for other platforms have been marked as incomplete.

with assertEquals

PG specific attribute PDO::PGSQL_ATTR_DISABLE_NATIVE_PREPARED_STATEMENT is now only set when it is still supported

Updated test for uppercased values

added literals off and on to the testcase

added postgresql platform conversion test

to test preg_replace performance

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.