N-(4-[2-(1,2,3,4-Tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine Carboxamide (GF120918) As a Chemical ATP-Binding Cassette Transporter Family G Member 2 (Abcg2) Knockout Model to Study Nitrofurantoin Transfer into Milk