message: Make JPEG encoder output valid images again.
  Fixes issue It looks like commit
  stopped emitting the DC table by accident, resulting in JPEG files missing
  huffman table 0.
diff: @@ -375,6 +375,10 @@ impl<'a, W: Write> JPEGEncoder<'a, W> {
  try!(self.writer.write_segment(DQT, Some(&buf)));
  }
  + build_huffman_segment(&mut buf, DCCLASS, LUMADESTINATION,
  + &STD_LUMA_DC_CODE_LENGTHS, &STD_LUMA_DC_VALUES);
  + try!(self.writer.write_segment(DHT, Some(&buf)));
  +
  build_huffman_segment(&mut buf, ACCLASS, LUMADESTINATION,
  &STD_LUMA_AC_CODE_LENGTHS, &STD_LUMA_AC_VALUES);
  try!(self.writer.write_segment(DHT, Some(&buf)));
  