message: fix string_lit_as_bytes
  this is cleaner
diff: @@ -688,7 +688,7 @@ impl<'a, W: Write> ImageEncoder for JpegEncoder<'a, W> {
  fn build_jfif_header(m: &mut Vec<u8>, density: PixelDensity) {
  m.clear();
  - m.extend_from_slice("JFIF".as_bytes());
  + m.extend_from_slice(b"JFIF");
  m.extend_from_slice(&[0, 0x01, 0x02,
  match density.unit {
  PixelDensityUnit::PixelAspectRatio => 0x00,
  