message: Fix pbm decoder decoding of ascii data
  Contrary to the binary decoder this would never get expanded in a post
  processing step so that it must produce the correct bytes from the
  start.
diff: @@ -736,7 +736,7 @@ impl Sample for PbmBit {
  let count = (width*height*samples) as usize;
  let raw_samples = reader.bytes()
  .filter_map(|ascii| match ascii {
  - Ok(b'0') => Some(Ok(1)),
  + Ok(b'0') => Some(Ok(255)),
  Ok(b'1') => Some(Ok(0)),
  Err(err) => Some(Err(ImageError::IoError(err))),
  Ok(b'\t')
  