Unit test test_invalid_chunk in Chapter 3 should add a standard header

[huyufeifei]

    #[test]
    fn test_invalid_chunk() {
        let mut chunk_bytes: Vec<u8> = testing_chunks()
            .into_iter()
            .flat_map(|chunk| chunk.as_bytes())
            .collect();

        #[rustfmt::skip]
        let mut bad_chunk = vec![
            0, 0, 0, 5,         // length
            32, 117, 83, 116,   // Chunk Type (bad)
            65, 64, 65, 66, 67, // Data
            1, 2, 3, 4, 5       // CRC (bad)
        ];

        chunk_bytes.append(&mut bad_chunk);

        let png = Png::try_from(chunk_bytes.as_ref());

        assert!(png.is_err());
    }

This is used to test invalid chunks, so I think it needs a correct standard header. I changed it to below:

    #[test]
    fn test_invalid_chunk() {
        let mut chunk_bytes: Vec<u8> = testing_chunks()
            .into_iter()
            .flat_map(|chunk| chunk.as_bytes())
            .collect();

        #[rustfmt::skip]
        let mut bad_chunk = vec![
            0, 0, 0, 5,         // length
            32, 117, 83, 116,   // Chunk Type (bad)
            65, 64, 65, 66, 67, // Data
            1, 2, 3, 4, 5       // CRC (bad)
        ];

        chunk_bytes.append(&mut bad_chunk);
        let bytes: Vec<u8> = [137, 80, 78, 71, 13, 10, 26, 10]
            .iter()
            .chain(chunk_bytes.iter())
            .copied()
            .collect();

        let png = Png::try_from(bytes.as_ref());

        assert!(png.is_err());
    }