Fixed cs issues

If you pass a null to the where array of update or delete it's
pretty obvious what you want.

Most (all?) databases won't match anything if you actually try
to compare with NULL making this a silent failure without it.

This mocks connect(), sets the internal _conn to an object, then calls close, which
should re-set _conn back to null. The getWrappedConnection() call gives us access
to _conn, but does NOT trigger a re-connect, since we mocked connect() to do nothing.
Ultimately, if _conn is unset, calling getWrappedConnection() will error. If _conn
is not set to null, the test will fail. This tests all the cases.

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.