@Test
    public void shouldReturnBook_increaseNumberOfCopies() {
        Book bookToTest = new Book("TestTitle", "2017", "Smith", 1);
        int expectedNumberOfCopies = 1;

        bookToTest.lendBook("Jones");
        bookToTest.returnBook("Jones");

        assertEquals(expectedNumberOfCopies, bookToTest.getNumberOfCopies());
    }