message: Enforce newline at end of first line of header
  This is the case even for unknown header versions, which can only alter
  anything after the first line.
diff: @@ -236,7 +236,9 @@ mod read {
  tag(AGE_MAGIC),
  alt((
  map(header_v1, Header::V1),
  - map(arbitrary_string, |s| Header::Unknown(s.to_string())),
  + map(terminated(arbitrary_string, newline), |s| {
  + Header::Unknown(s.to_string())
  + }),
  )),
  )(input)
  }
  