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.
Merge pull request #623 from jgallred/remove-extra-pdo-reference

Prevent Connection from maintaining a second reference to an injected PDO object.